Fix flaky toBlob tests

Some layout tests are flaky on WebKit Leak bot due to m_document introduced to
CanvasAsyncBlobCreator in https://codereview.chromium.org/2143353002.

TBR=haraken@chromium.org
BUG=628308

Review-Url: https://codereview.chromium.org/2149233004
Cr-Commit-Position: refs/heads/master@{#405622}

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

@@ -112,6 +112,7 @@ void CanvasAsyncBlobCreator::dispose()
     // Eagerly let go of references to prevent retention of these
     // resources while any remaining posted tasks are queued.
     m_data.clear();
+    m_document.clear();
     m_callback.clear();
 }