Disable flaky FrameImplTest.PostMessage test

Chromium sheriff.

TBR=wez@chromium.org

Bug: 907717
Change-Id: Id0a5bb81ed623572e87f19b9229048febdd3ff2c
Reviewed-on: https://chromium-review.googlesource.com/c/1347642Reviewed-by: Hayato Ito <hayato@chromium.org>
Commit-Queue: Hayato Ito <hayato@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610299}

webrunner/browser/frame_impl_browsertest.cc

@@ -781,7 +781,8 @@ class AsyncValueReceiver {
   DISALLOW_COPY_AND_ASSIGN(AsyncValueReceiver<T>);
 };
 
-IN_PROC_BROWSER_TEST_F(FrameImplTest, PostMessage) {
+// Flaky. https://crbug.com/907717
+IN_PROC_BROWSER_TEST_F(FrameImplTest, DISABLED_PostMessage) {
   chromium::web::FramePtr frame = CreateFrame();
 
   ASSERT_TRUE(embedded_test_server()->Start());