Implement WebContentsView::GetDropData for Mac; disable on Aura

I don't know what the appropriate analog is for this UI on Aura, so just disabling it now.

BUG=55584
TBR=jam@chromium.org

Review URL: https://chromiumcodereview.appspot.com/10386014

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@135785 0039d316-1c4b-4281-b951-d872f2087c98
6 files changed