Mac Support for drag-and-drop tests that start dragging via mouse simulation.

Mac Support for drag-and-drop tests that start dragging via mouse simulation.

This CL:

1. Adds Mac support for drag-and-drop tests that start dragging via
   mouse simulation.  This is done by changing how drag-start event
   is intercepted, so that this is done in an Aura-agnostic way
   (by adding a test hook to RenderWidgetHostImpl::OnStartDragging).

   The new test hook give opportunity for more test verification
   (i.e. verification of img.alt text) although verification of
   drag start location is now only done via DOMDragEventVerifier
   (before this CL it was also compared with location reported
   via WaitUntilDragStart).

2. Moves the remainining Aura-specific functionality (for simulating
   external drag entering a browser window) into
   drag_and_drop_interactive_uitest_aura.cc (and disabling
   DropTextFromOutside test on Mac).

BUG=647249

[Łukasz Anforowicz, 2017-01-09 22:17:23]

The CQ bit was checked by lukasza@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-01-09 22:18:18]

Dry run: CQ is trying da patch. Follow status at
 
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[commit-bot: I haz the power, 2017-01-09 23:17:12]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-01-09 23:17:13]

Dry run: Try jobs failed on following builders:
  win_chromium_compile_dbg_ng on master.tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_comp...)