Move fragment navigation start to FrameLoader::load(), remove some other dead code there.

Also, inline a simple helper in shouldPerformFragmentNavigation.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/19830002

git-svn-id: svn://svn.chromium.org/blink/trunk@154766 bbb929c8-8fbe-4397-9dbb-9b2b20218538