Remove FrameLoaderStateMachine::DisplayingInitialEmptyDocumentPostCommit

Jump straight to CommittedFirstRealLoad instead. Something similar was
attempted and rolled out due to a perf regression a few months ago. This
version passes dromaeo_domcoreattr, which was what regressed.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/22891017

git-svn-id: svn://svn.chromium.org/blink/trunk@156502 bbb929c8-8fbe-4397-9dbb-9b2b20218538