Handle a null context in CanvasAsyncBlobCreator constructor

Bug: 1061309
Test: fast/canvas/toBlob-in-detached-doument.html
Change-Id: I3d515666c9228d6d32791bad588149f879020b2e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2103046Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Nate Chapin <japhet@chromium.org>
Cr-Commit-Position: refs/heads/master@{#750299}

third_party/blink/renderer/core/html/canvas/canvas_async_blob_creator.cc

@@ -265,7 +265,7 @@ CanvasAsyncBlobCreator::CanvasAsyncBlobCreator(
 
   idle_task_status_ = kIdleTaskNotSupported;
   num_rows_completed_ = 0;
-  if (context->IsDocument()) {
+  if (context && context->IsDocument()) {
     parent_frame_task_runner_ =
         context->GetTaskRunner(TaskType::kCanvasBlobSerialization);
   }

third_party/blink/web_tests/fast/canvas/toBlob-in-detached-document-expected.txt

+PASS if no crash.

third_party/blink/web_tests/fast/canvas/toBlob-in-detached-document.html

+PASS if no crash.
+<script>
+if (window.testRunner)
+  testRunner.dumpAsText();
+
+new Document().createElementNS('http://www.w3.org/1999/xhtml', 'canvas').toBlob(() => {});
+</script>