Use RenderProcessHost::Shutdown in test to avoid flakiness.

Speculative fix for timeouts on linux_android_rel_ng in the
FrameTreeBrowserTest.FrameTreeAfterCrash test.

BUG=727725
TEST=Less flakiness
CQ_INCLUDE_TRYBOTS=master.tryserver.chromium.linux:linux_site_isolation

Review-Url: https://codereview.chromium.org/2919483003
Cr-Commit-Position: refs/heads/master@{#476362}

content/browser/frame_host/frame_tree_browsertest.cc

@@ -135,7 +135,8 @@ IN_PROC_BROWSER_TEST_F(FrameTreeBrowserTest, FrameTreeAfterCrash) {
   RenderProcessHostWatcher crash_observer(
       shell()->web_contents(),
       RenderProcessHostWatcher::WATCH_FOR_PROCESS_EXIT);
-  NavigateToURL(shell(), GURL(kChromeUICrashURL));
+  ASSERT_TRUE(
+      shell()->web_contents()->GetMainFrame()->GetProcess()->Shutdown(0, true));
   crash_observer.Wait();
 
   // The frame tree should be cleared.