Fix test that crashes when we create a new frame on every sub-frame nav.

DevToolsExtensionTest.HttpIframeInDevToolsExtensionPanel crashes when
RenderDocument is set to subframe.

Bug: 1064944
Change-Id: I6a17a389a15ee20b10b4ed87f4378842bee13ab9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2145737Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Andrey Kosyakov <caseq@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#758699}

chrome/browser/devtools/devtools_sanity_browsertest.cc

@@ -955,6 +955,8 @@ IN_PROC_BROWSER_TEST_F(DevToolsExtensionTest,
   ASSERT_TRUE(content::ExecuteScript(web_frame_rfh, about_blank_javascript));
 
   web_about_blank_manager.WaitForNavigationFinished();
+  // After navigation, the frame may change.
+  web_frame_rfh = ChildFrameAt(panel_frame_rfh, 2);
 
   EXPECT_EQ(about_blank_url, web_frame_rfh->GetLastCommittedURL());
   EXPECT_EQ(web_url.host(),