mac: route mouse up and down event to views that overlay content area

A view that overlays a content area (hosted in a view::WebView) cannot
receive mouse up and down events because all events go to WebView's
NSView. This NSView is above the NSWindow' contentView that hosts the
toolkit-views tree.

To fix the issue, this CL redirects such events to the content NSView
which hosts the root toolkit-views tree by overriding hitTest: in
BridgedContentView.

Implementation wise, this CL adds mojo method GetHitTestResult() that
replaces the old GetIsDraggableBackgroundAt(). The method returns
whether the point is within the draggable area, a regular view, or a
NativeViewHost that hosts a subview of the contentView.

Note that mouse move will still be sent to all NSViews, including the
content NSView and all subviews. When RenderWidgetHostViewCocoa receives
mouse moves events, it uses hitTest: to check if the event should be
sent to blink and decide if mouse enter and exit events should be
synthesized. This is the existing behavior.

Bug: 417607649
Change-Id: I10728baecda999bfd451bc8fb98fbcd1c94a75fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6574464
Commit-Queue: Keren Zhu <kerenzhu@chromium.org>
Reviewed-by: Joe Mason <joenotcharles@google.com>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1466557}