Reland "Make same-document history navigations commit with the proper type."

This reverts commit 9f7dd81e9620e67024b43cedffd25a35c5cd2014.

There is a quirk in HistoryController where we commit a history navigation
immediately upon starting a same-document navigation. If a same-document
history navigation is committed with a HistoryCommitType of BackForwardCommit
instead of HistoryInertCommit, the commit logic does the right thing and we
don't need that quirk.

BUG=

Review URL: https://codereview.chromium.org/263903006

git-svn-id: svn://svn.chromium.org/blink/trunk@174110 bbb929c8-8fbe-4397-9dbb-9b2b20218538