Avoid corrupting the pending_entry when an unexpected navigation commits.

      BUG=68350
TEST=TabContentsTest.CrossSiteNavigationBackPreempted
TEST=NavigationControllerTest.RestoreNavigate

Review URL: http://codereview.chromium.org/6709056

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@79028 0039d316-1c4b-4281-b951-d872f2087c98