Revert of Simplify CanvasAsyncBlobCreator by removing ContextObserver and related tests

third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp

Index: third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
diff --git a/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp b/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
index 63d691d071e28e0340a84d5aae3a19f0c0839ca3..2b554b54362aa1de55080ff79e61bf09aad4937e 100644
--- a/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
@@ -580,7 +580,7 @@
     return toDataURLInternal(mimeType, quality, BackBuffer);
 }
 
-void HTMLCanvasElement::toBlob(BlobCallback* callback, const String& mimeType, const ScriptValue& qualityArgument, ExceptionState& exceptionState)
+void HTMLCanvasElement::toBlob(ScriptState* scriptState, BlobCallback* callback, const String& mimeType, const ScriptValue& qualityArgument, ExceptionState& exceptionState)
 {
     if (!originClean()) {
         exceptionState.throwSecurityError("Tainted canvases may not be exported.");
@@ -609,9 +609,9 @@
     // Add a ref to keep image data alive until completion of encoding
     RefPtr<DOMUint8ClampedArray> imageDataRef(imageData->data());
 
-    RefPtr<CanvasAsyncBlobCreator> asyncCreatorRef = CanvasAsyncBlobCreator::create(imageDataRef.release(), encodingMimeType, imageData->size(), callback);
-
-    if (encodingMimeType == DefaultMimeType) {
+    RefPtr<CanvasAsyncBlobCreator> asyncCreatorRef = CanvasAsyncBlobCreator::create(imageDataRef.release(), encodingMimeType, imageData->size(), callback, scriptState->executionContext());
+
+    if (Platform::current()->isThreadedCompositingEnabled() && (encodingMimeType == DefaultMimeType)) {
         asyncCreatorRef->scheduleAsyncBlobCreation(true);
     } else {
         asyncCreatorRef->scheduleAsyncBlobCreation(false, quality);