Clean up ResourceHandler API.

1. Move URLRequest member to ResourceHandler base class and add convenience functions for subclasses.
2. Make OnWillRead take a scoped_refptr to avoid leaking the buffer.

BUG=295239
TEST=No behavior change.

Review URL: https://codereview.chromium.org/25536005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@227318 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 227318