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This modifies 3 parts of WebContentsViewMac and its friends.

1: Initiation of a drag from web contents
This action follows the following path
  * WebContentsViewMac::StartDragging is called, which calls
  * -[WebContentsViewCocoa startDragWithDropData...], which
  * Creates a WebDragSource (to store state for the drag) and calls
  * -[WebDragSource startDrag], which calls
  * -[NSWindow dragImage...]
This doesn't work across processes because the WebContentsViewMac
now exists in the browser process and the WebContentsViewCocoa
exists in the app shim process. Fix this by adding the mojo method
WebContentsNSViewBridge::StartDrag, which is called by
WebContentsViewMac::StartDragging in the browser process, and makes
the -[WebContentsViewCocoa startDragWithDropData...] when it is
received in the app shim process.

2: Completion of a drag via -[NSDraggingSource draggedImage...]
This method takes the following path
  * -[WebContentsViewCocoa(NSDraggingSource) draggedImage...] is
    called by the system, which then calls
  * -[WebDragSource endDragAt:], which then calls
  * WebContentsImpl::SystemDragEnded and
    WebContentsImpl::DragSourceEndedAt
This doesn't work across processes because the WebContentsViewCocoa
is in the app shim process while the WebContentsImpl is in the browser
process. Fix this by adding the mojo method
WebContentsNSViewClient::EndDrag, which is called by WebDragSource
in the app shim process, and makes the WebContentsImpl calls when
it is received in the browser process.

3: -[NSDraggingInfo namesOfPromisedFilesDroppedAtDestination] is called
This method takes the following path
  * -[WebContentsViewCocoa(NSDraggingInfo) namesOfPromised...] calls
  * -[WebDragSource dragPromisedFileTo...], which creates a
  * DragDownloadFile, which uses the WebContentsImpl
Same as the above, the WebContentsViewCocoa cannot access the
WebContentsImpl, so we add a mojo method to bridge between the two.

Move tracking of state for the drag (e.g, the source RenderWidgetHost)
from WebDragSource to WebContentsViewMac.

This eliminates the last need for accessing the WebContentsViewMac
in WebContentsViewCocoa and WebDragSource, to remove the related
member variables.

Because the use of base::ThreadRestrictions::ScopedAllowIO is moved
by this patch, update it to be ScopedAllowBlocking and add
WebContentsViewMac as a friend of ScopedAllowBlocking.

Bug: 898608
Change-Id: I4e7f069e96902b7f0ef26e8579444be96b765a20
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1497543Reviewed-by: danakj <danakj@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: ccameron <ccameron@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637841}