Fix flaky toBlob tests

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

Index: third_party/WebKit/Source/core/html/canvas/CanvasAsyncBlobCreator.cpp
diff --git a/third_party/WebKit/Source/core/html/canvas/CanvasAsyncBlobCreator.cpp b/third_party/WebKit/Source/core/html/canvas/CanvasAsyncBlobCreator.cpp
index 16f6e891adf10db10b0d1793b6fa1bbbde30230b..094e80257b54911f076f9bc52af6f0057b2950be 100644
--- a/third_party/WebKit/Source/core/html/canvas/CanvasAsyncBlobCreator.cpp
+++ b/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();
 }