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

TEST=covered by test

Change-Id: I3c350d51ba0258dafc70fcee98c694b7fc39ce0b
Commit-Queue: Scott Violet <>
Reviewed-by: Xiyuan Xia <>
Cr-Commit-Position: refs/heads/master@{#622096}
8 files changed