Use CanvasBlobSerialization task runner for canvas serialization

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 e5c53023e278cb7e184deca631af9ea78433224e..af22dd38d819cd96b0b869a35d1f96098dd5c404 100644
--- a/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
+++ b/third_party/WebKit/Source/core/html/HTMLCanvasElement.cpp
@@ -713,7 +713,7 @@ void HTMLCanvasElement::toBlob(BlobCallback* callback, const String& mimeType, c
 
     if (!isPaintable()) {
         // If the canvas element's bitmap has no pixels
-        TaskRunnerHelper::getUnthrottledTaskRunner(&document())->postTask(BLINK_FROM_HERE, WTF::bind(&BlobCallback::handleEvent, wrapPersistent(callback), nullptr));
+        TaskRunnerHelper::get(TaskType::CanvasBlobSerialization, &document())->postTask(BLINK_FROM_HERE, WTF::bind(&BlobCallback::handleEvent, wrapPersistent(callback), nullptr));
         return;
     }