aura: Make OSExchangeDataProvider::HasURL() take a policy.

Starting in r244538, we started passing a policy about whether we should
treat filenames as a valid value in GetURLAndTitle(). However, we didn't
pass that policy through to HasURL(), and the two being out of sync
meant that valid drag targets like the omnibox or select file buttons in
the content area wouldn't accept files.

BUG=345032, 339268

Review URL: https://codereview.chromium.org/173173002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252438 0039d316-1c4b-4281-b951-d872f2087c98
Remove unused headers.

These files include ui/compositor/test/test_context_factory.h but
do not use it, and the file is going away.

TBR=ben@chromium.org,piman@chromium.org
BUG=270918

Review URL: https://codereview.chromium.org/174243005

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252439 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252438
SVN changes up to revision 252439