Prevent <canvas> destructor from causing nested garbage collection

In the destruction of HTMLCanvasElement, we call
HTMLCanvasElement::existingDrawingContext(), whose behavior was changed
incorrectly in r162330 and was triggering the allocation of an ImageBuffer
during tear-down.  This is not only wasteful, but it can trigger a
garbage collection because the creation of an ImageBuffer will increase
the amount of externallay allocated memory that is declared to V8.
Triggering a GC during the finalization of a garbage collected object
is bad.

BUG=378289

Review URL: https://codereview.chromium.org/303803002

git-svn-id: svn://svn.chromium.org/blink/trunk@174984 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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

@@ -572,10 +572,8 @@ GraphicsContext* HTMLCanvasElement::drawingContext() const
 
 GraphicsContext* HTMLCanvasElement::existingDrawingContext() const
 {
-    if (m_didFailToCreateImageBuffer) {
-        ASSERT(!hasImageBuffer());
+    if (!hasImageBuffer())
         return 0;
-    }
 
     return drawingContext();
 }