DevTools: fix CSS.addRule protocol method for file:// urls

The previous attempt to fix the bug was in crrev.com/539166:
the patch allowed adding rules to the inspector style sheets.
However, it didn't take into account adding rules into the
already-existing style sheets.

This patch fixes CSS.addRule altogether, allowing CSSOM access
if the operation is happenning from-inside InspectorMutationScope.

Our tests didn't catch the regression since we run with
--allow-file-access-from-files flag.

BUG=818518
R=dgozman

Change-Id: Ic6b198a5ef7abbabcef671edbefb5a9b14fde531
Reviewed-on: https://chromium-review.googlesource.com/1018526
Commit-Queue: Andrey Lushnikov <lushnikov@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#552161}

third_party/blink/renderer/core/css/css_style_sheet.cc

@@ -238,6 +238,23 @@ void CSSStyleSheet::DidMutate() {
         ownerNode()->GetTreeScope());
 }
 
+void CSSStyleSheet::EnableRuleAccessForInspector() {
+  enable_rule_access_for_inspector_ = true;
+}
+void CSSStyleSheet::DisableRuleAccessForInspector() {
+  enable_rule_access_for_inspector_ = false;
+}
+
+CSSStyleSheet::InspectorMutationScope::InspectorMutationScope(
+    CSSStyleSheet* sheet)
+    : style_sheet_(sheet) {
+  style_sheet_->EnableRuleAccessForInspector();
+}
+
+CSSStyleSheet::InspectorMutationScope::~InspectorMutationScope() {
+  style_sheet_->DisableRuleAccessForInspector();
+}
+
 void CSSStyleSheet::ReattachChildRuleCSSOMWrappers() {
   for (unsigned i = 0; i < child_rule_cssom_wrappers_.size(); ++i) {
     if (!child_rule_cssom_wrappers_[i])
@@ -299,6 +316,8 @@ void CSSStyleSheet::ClearOwnerNode() {
 }
 
 bool CSSStyleSheet::CanAccessRules() const {
+  if (enable_rule_access_for_inspector_)
+    return true;
   if (is_inline_stylesheet_)
     return true;
   KURL base_url = contents_->BaseURL();
@@ -307,8 +326,6 @@ bool CSSStyleSheet::CanAccessRules() const {
   Document* document = OwnerDocument();
   if (!document)
     return true;
-  if (document->GetStyleEngine().InspectorStyleSheet() == this)
-    return true;
   if (document->GetSecurityOrigin()->CanReadContent(base_url))
     return true;
   if (allow_rule_access_from_origin_ &&

third_party/blink/renderer/core/css/css_style_sheet.h

@@ -152,6 +152,21 @@ class CORE_EXPORT CSSStyleSheet final : public StyleSheet {
   void DidMutateRules();
   void DidMutate();
 
+  class InspectorMutationScope {
+    STACK_ALLOCATED();
+
+   public:
+    explicit InspectorMutationScope(CSSStyleSheet*);
+    ~InspectorMutationScope();
+
+   private:
+    Member<CSSStyleSheet> style_sheet_;
+    DISALLOW_COPY_AND_ASSIGN(InspectorMutationScope);
+  };
+
+  void EnableRuleAccessForInspector();
+  void DisableRuleAccessForInspector();
+
   StyleSheetContents* Contents() const { return contents_.Get(); }
 
   bool IsInline() const { return is_inline_stylesheet_; }
@@ -199,6 +214,7 @@ class CORE_EXPORT CSSStyleSheet final : public StyleSheet {
   // This alternate variable is only used for constructed CSSStyleSheet.
   // For other CSSStyleSheet, consult the alternate attribute.
   bool alternate_from_constructor_ = false;
+  bool enable_rule_access_for_inspector_ = false;
   String title_;
   scoped_refptr<MediaQuerySet> media_queries_;
   MediaQueryResultList viewport_dependent_media_query_results_;
@@ -219,8 +235,7 @@ class CORE_EXPORT CSSStyleSheet final : public StyleSheet {
 
 inline CSSStyleSheet::RuleMutationScope::RuleMutationScope(CSSStyleSheet* sheet)
     : style_sheet_(sheet) {
-  if (style_sheet_)
-    style_sheet_->WillMutateRules();
+  style_sheet_->WillMutateRules();
 }
 
 inline CSSStyleSheet::RuleMutationScope::RuleMutationScope(CSSRule* rule)

third_party/blink/renderer/core/inspector/InspectorCSSAgent.cpp

@@ -622,10 +622,12 @@ class InspectorCSSAgent::AddRuleAction final
   }
 
   bool Undo(ExceptionState& exception_state) override {
+    CSSStyleSheet::InspectorMutationScope scope(style_sheet_->PageStyleSheet());
     return style_sheet_->DeleteRule(added_range_, exception_state);
   }
 
   bool Redo(ExceptionState& exception_state) override {
+    CSSStyleSheet::InspectorMutationScope scope(style_sheet_->PageStyleSheet());
     css_rule_ = style_sheet_->AddRule(rule_text_, location_, &added_range_,
                                       exception_state);
     if (exception_state.HadException())