Oilpan: Use CrossThreadPersistent for CanvasAsyncBlobCreator::m_callback

The image encoder runs on a background worker thread, which is not attached to Oilpan.
Thus CanvasAsyncBlobCreator::m_callback touched by the image encoder must use CrossThreadPersistent.

BUG=581877

Review URL: https://codereview.chromium.org/1641743005

Cr-Commit-Position: refs/heads/master@{#372053}

third_party/WebKit/Source/core/html/canvas/CanvasAsyncBlobCreator.h

@@ -42,7 +42,7 @@ private:
     const IntSize m_size;
     size_t m_pixelRowStride;
     const String m_mimeType;
-    Persistent<BlobCallback> m_callback;
+    CrossThreadPersistent<BlobCallback> m_callback;
 
     RefPtr<CanvasAsyncBlobCreator> m_selfRef;
     void clearSelfReference();