[PDF Ink Signatures] Refactor PDFiumEngine::ReadOnly()

Refactor by moving the form highlight and selection logic into separate
helper methods. In https://crrev.com/c/5841317, these helper methods
will be reused to remove form highlights and text selection when
entering annotation mode.

Bug: 353923998
Change-Id: Ic1424a4b967b29f27a6f24045b9d044f53132dad
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5841316
Commit-Queue: Andy Phan <andyphan@chromium.org>
Code-Coverage: findit-for-me@appspot.gserviceaccount.com <findit-for-me@appspot.gserviceaccount.com>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1352366}

pdf/pdfium/pdfium_engine.cc

@@ -2174,9 +2174,13 @@ bool PDFiumEngine::IsReadOnly() const {
 
 void PDFiumEngine::SetReadOnly(bool enable) {
   read_only_ = enable;
+  SetFormHighlight(!read_only_);
+  ClearTextSelection();
+}
 
+void PDFiumEngine::SetFormHighlight(bool enable_form) {
   // Restore form highlights.
-  if (!read_only_) {
+  if (enable_form) {
     FPDF_SetFormFieldHighlightAlpha(form(), kFormHighlightAlpha);
     return;
   }
@@ -2184,8 +2188,9 @@ void PDFiumEngine::SetReadOnly(bool enable) {
   // Hide form highlights.
   FPDF_SetFormFieldHighlightAlpha(form(), /*alpha=*/0);
   KillFormFocus();
+}
 
-  // Unselect text.
+void PDFiumEngine::ClearTextSelection() {
   SelectionChangeInvalidator selection_invalidator(this);
   selection_.clear();
 }

pdf/pdfium/pdfium_engine.h

@@ -850,6 +850,11 @@ class PDFiumEngine : public DocumentLoader::Client, public IFSDK_PAUSE {
   // requests the thumbnail for that page.
   void MaybeRequestPendingThumbnail(int page_index);
 
+  // Sets whether form highlight should be enabled or cleared.
+  void SetFormHighlight(bool enable_form);
+
+  void ClearTextSelection();
+
   const raw_ptr<PDFiumEngineClient> client_;
 
   // The current document layout.