[Android] UrlUtilities#fixUrl(...) crash fix on invalid string argument.

Check if the GURL instance is valid before calling GURL::spec().
This fixes DCHECK crash.

BUG=318901

Review URL: https://codereview.chromium.org/104653005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@238847 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 238847