pushState should not be treated as a navigation (step 1 of 3)

Add a new version of RenderViewImpl::didStartLoading() that takes a boolean
indicating whether the navigation is to the same document or a different
document. blink will call this new version in a later patch.

BUG=50298

Review URL: https://codereview.chromium.org/102563003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@243122 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 243122