Fix Linux Aura drag-and-drop cancellation.

Fixes Escape key drag-and-drop cancellation handling.
(this is a followup fix for Issue 344289 and r254289)

SendXdndLeave(source_current_window_) in OnMoveLoopEnded.
(seems like a misnomer, this is the target drop window)
(alerts the target window to repaint and cleanup, etc.)

BUG=349151,344289
TEST=Dragging text, bookmarks, etc. from a Linux Aura browser window to the same window, other Chrome windows, and other application is cancelled correctly when pressing [Esc] (no lingering drop indicator, etc.).
R=erg@chromium.org

Review URL: https://codereview.chromium.org/181013011

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255449 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255449