Never fire popstate event when navigating between documents

The specification says to only fire popstate events when the document's
"latest entry" isn't the session history entry being navigated to. This
can be the case when navigating between documents (when skipping history
entries, e.g. using history.go(-2)), but this patch simply disables
popstate entirely when navigating between documents.

The resulting behavior matches that of Firefox and MSIE.

BUG=400427

Review URL: https://codereview.chromium.org/680273004

git-svn-id: svn://svn.chromium.org/blink/trunk@184628 bbb929c8-8fbe-4397-9dbb-9b2b20218538