Disable flaky test ContextImplTest.Stop().

This test is flaking on the Fuchsia bot, e.g.
https://ci.chromium.org/p/chromium/builders/luci.chromium.ci/Fuchsia%20x64/16873

TBR=kmarshall@chromium.org

Bug: 876894
Change-Id: I455c56835af050aa1f9ff6715a6243500cc06e1d
Reviewed-on: https://chromium-review.googlesource.com/1187552Reviewed-by: Scott Little <sclittle@chromium.org>
Commit-Queue: Scott Little <sclittle@chromium.org>
Cr-Commit-Position: refs/heads/master@{#585668}

webrunner/browser/context_impl_browsertest.cc

@@ -519,7 +519,8 @@ struct WebContentsObserverForStop : public content::WebContentsObserver {
   MOCK_METHOD0(NavigationStopped, void());
 };
 
-IN_PROC_BROWSER_TEST_F(ContextImplTest, Stop) {
+// TODO(crbug.com/876894): Disabled due to flakiness.
+IN_PROC_BROWSER_TEST_F(ContextImplTest, DISABLED_Stop) {
   chromium::web::FramePtr frame = CreateFrame();
 
   chromium::web::NavigationControllerPtr controller;