Simplify FrameLoaderClient interface to take HTMLFormElement* instead of FormState
when only FormState::form() was used.

R=tkent@chromium.org
BUG=none
TEST=compile

Review URL: https://codereview.chromium.org/198543002

git-svn-id: svn://svn.chromium.org/blink/trunk@169109 bbb929c8-8fbe-4397-9dbb-9b2b20218538