RenderDocument: Update tests KeyboardLockBrowserTest

With RenderDocument:
https://docs.google.com/document/d/1C2VKkFRSc0kdmqjKan1G4NlNlxWZqE4Wam41FNMgnmA/edit#
a new RenderFrameHost is used after every navigations, not only the
cross-process ones, but also the same-process ones.

This CL updates tests so that they don't assume the RenderFrameHost to
be preserved.

Bug: 936696
Change-Id: I222e6a76cb9d16b68cb4802a09bc41ebd9d1bd9c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1538295
Commit-Queue: Arthur Sonzogni <arthursonzogni@chromium.org>
Reviewed-by: Alex Moshchuk <alexmos@chromium.org>
Cr-Commit-Position: refs/heads/master@{#645280}

content/browser/keyboard_lock_browsertest.cc

@@ -816,15 +816,15 @@ IN_PROC_BROWSER_TEST_F(KeyboardLockBrowserTest,
   ASSERT_TRUE(web_contents()->GetKeyboardLockWidget());
   ASSERT_FALSE(web_contents()->GetRenderWidgetHostView()->IsKeyboardLocked());
 
-  RenderFrameHost* main_frame = web_contents()->GetMainFrame();
-  RenderFrameHost* child = ChildFrameAt(main_frame, 2);
-  ASSERT_TRUE(child);
-
   // The third child is cross-domain and has the allowfullscreen attribute set.
   ASSERT_TRUE(
       NavigateIframeToURL(web_contents(), kChildIframeName_2,
                           https_test_server()->GetURL(kCrossSiteChildDomain2,
                                                       kFullscreenFramePath)));
+  RenderFrameHost* main_frame = web_contents()->GetMainFrame();
+  RenderFrameHost* child = ChildFrameAt(main_frame, 2);
+  ASSERT_TRUE(child);
+
   ASSERT_TRUE(ExecuteScript(child, "activateFullscreen()"));
 
   ASSERT_EQ(main_frame->GetView()->GetRenderWidgetHost(),
@@ -839,15 +839,15 @@ IN_PROC_BROWSER_TEST_F(KeyboardLockBrowserTest,
   // frame activates fullscreen.
   NavigateToTestURL(https_cross_site_frame());
 
-  RenderFrameHost* main_frame = web_contents()->GetMainFrame();
-  RenderFrameHost* child = ChildFrameAt(main_frame, 2);
-  ASSERT_TRUE(child);
-
   // The third child is cross-domain and has the allowfullscreen attribute set.
   ASSERT_TRUE(
       NavigateIframeToURL(web_contents(), kChildIframeName_2,
                           https_test_server()->GetURL(kCrossSiteChildDomain2,
                                                       kFullscreenFramePath)));
+  RenderFrameHost* main_frame = web_contents()->GetMainFrame();
+  RenderFrameHost* child = ChildFrameAt(main_frame, 2);
+  ASSERT_TRUE(child);
+
   ASSERT_TRUE(ExecuteScript(child, "activateFullscreen()"));
 
   RequestKeyboardLock(FROM_HERE);