Make maybeSavePassword() a top-level method in BrowserFrame.

We need to be able to reuse this logic from the Chrome HTTP
stack. This is just a refactoring that doesn't change any
behaviour; we'll need a separate change in external/webkit
to call this from the code that handles POST requests.

Change-Id: I3f3f84a15a8501f63de974bffa6f7c911a3d3820