Always create a PDFEngine in OutOfProcessInstance::Init()

Revival of crrev.com/c/1931439 by tsepez@.

Init() is only called on newly created instances, so there's no way for
|engine_| to be created before Init() is called. Instead of
conditionalizing the creation of |engine_|, assert that it hasn't been
created yet.

Bug: 852273
Change-Id: Id919a9fc9bcf23ee8d66e81fbfd745c164da87cf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2207740
Commit-Queue: Daniel Hosseinian <dhoss@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#773493}

pdf/out_of_process_instance.cc

@@ -455,6 +455,8 @@ OutOfProcessInstance::~OutOfProcessInstance() {
 bool OutOfProcessInstance::Init(uint32_t argc,
                                 const char* argn[],
                                 const char* argv[]) {
+  DCHECK(!engine_);
+
   pp::Var document_url_var = pp::URLUtil_Dev::Get()->GetDocumentURL(this);
   if (!document_url_var.is_string())
     return false;
@@ -518,9 +520,7 @@ bool OutOfProcessInstance::Init(uint32_t argc,
   if (!stream_url)
     stream_url = original_url;
 
-  if (!engine_) {
-    engine_ = PDFEngine::Create(this, enable_javascript);
-  }
+  engine_ = PDFEngine::Create(this, enable_javascript);
 
   // If we're in print preview mode we don't need to load the document yet.
   // A |kJSResetPrintPreviewModeType| message will be sent to the plugin letting