Prevent DOS of Autofill by limiting the number of form fields we send across from the renderer.

      BUG=76034
TEST=manual; see bug

Review URL: http://codereview.chromium.org/6670114

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@80098 0039d316-1c4b-4281-b951-d872f2087c98