chromeos: fixs dropping on webcontents

Windows that want to receive drop events must call
WindowTree::SetCanAcceptDrops(). This was only done for DesktopNativeWidgetAura's
content-window by DesktopWindowTreeHostMus. A better approach is to implicitly
call SetCanAcceptDrops() any time a DragDropDelegate is registered on a window.
Going this route means DesktopWindowTreeHostMus doesn't need to explicitly
call SetCanAcceptDrops() because DesktopNativeWidgetAura registers a
DropDropDelegate.

BUG=920789
TEST=covered by test

Change-Id: I3c350d51ba0258dafc70fcee98c694b7fc39ce0b
Reviewed-on: https://chromium-review.googlesource.com/c/1407082
Commit-Queue: Scott Violet <sky@chromium.org>
Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#622096}
8 files changed