Reset DocumentLoader::m_request's method to GET for pushState and replaceState

We currently correctly send a GET in the POST->replaceState->reload case, but
DocumentLoader::m_request still shows a method of POST, and as a result we
incorrectly report it as a POST to the browser process. This contributes to
unnecessarily showing a form resubmission warning in that case.

BUG=138324

Review URL: https://codereview.chromium.org/302543008

git-svn-id: svn://svn.chromium.org/blink/trunk@175011 bbb929c8-8fbe-4397-9dbb-9b2b20218538