Simplify starting a same-document navigation

* Don't bother checking navigation policy for a same document navigation, since the answer will always be yes and we already skip calling out to the embedder.

* Remove FrameLoader's checkNavigationPolicyAndContinueFragmentScroll() and spread it across load() (which calls it) and loadInSameDocument (which it calls).

* Use the UpdateBackForwardListPolicy as a parameter to loadInSameDocument instead of the boolean isNewNavigation.

* Properly set ClientRedirectPolicy in NavigationScheduler::scheduleLocationChange, so that loadInSameDocument doesn't have to patch it up later.

Review URL: https://codereview.chromium.org/126453005

git-svn-id: svn://svn.chromium.org/blink/trunk@164887 bbb929c8-8fbe-4397-9dbb-9b2b20218538