[MacViews] Enable dragging a window by its caption/draggable areas.
This works by making all subviews of [NSWindow contentView] draggable.
I.e. they return YES for mouseDownCanMoveWindow. The content view
itself is initially non-draggable. BridgedNativeWidget watches for any
mouse-downs and tests whether they should drag the window. If so, it
makes the content view draggable and reposts the event.
The event is reposted at the kCGSessionEventTap level because window
dragging seems to be handled there before the event is sent to the
application.
When BridgedNativeWidget sees the reposted mouse-down, window dragging
will have started, so the content view can be made non-draggable again.
BUG=485394,459877
Review URL: https://codereview.chromium.org/
1146873002
Cr-Commit-Position: refs/heads/master@{#333410}