Fixed test by changing the way a subframe was added.

Test NavigatorTestWithBrowserSideNavigation.BeginNavigation was broken
due to the way the subframe used was added to the tree. By directly
calling FrameTree::AddFrame the proper observer notifications were not
being executed and triggered a sanity check. It was replaced with the
already test-tailored TestRenderFrameHost::AppendChild.

BUG=455484

Review URL: https://codereview.chromium.org/904673004

Cr-Commit-Position: refs/heads/master@{#315063}

content/browser/frame_host/navigator_impl_unittest.cc

@@ -133,10 +133,7 @@ TEST_F(NavigatorTestWithBrowserSideNavigation, BeginNavigation) {
 
   // Add a subframe.
   FrameTreeNode* root_node = contents()->GetFrameTree()->root();
-  TestRenderFrameHost* subframe_rfh =
-      static_cast<TestRenderFrameHost*>(contents()->GetFrameTree()->AddFrame(
-          root_node, root_node->current_frame_host()->GetProcess()->GetID(), 14,
-          "Child"));
+  TestRenderFrameHost* subframe_rfh = main_test_rfh()->AppendChild("Child");
   ASSERT_TRUE(subframe_rfh);
 
   FrameTreeNode* subframe_node = subframe_rfh->frame_tree_node();