Don't create pending entries when a navigation is initiated by the page.

      If the page reloads while such a navigation happens, we could end up with the wrong pending entry.
Also make sure TestTabContents::NavigateAndCommit() does commit on the right RVH.

BUG=51680
TEST=See bug for steps.

Review URL: http://codereview.chromium.org/3257002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@58261 0039d316-1c4b-4281-b951-d872f2087c98