Mojo-ify calls from WebContentsViewCocoa to WebContentsViewMac

WebContentsViewCocoa is in the app shim process and WebContentsViewMac
is in the browser process, so calls from one to the other must go
through mojo.

Move WebDragDest from being owned by WebContentsViewCocoa to being
owned by WebContentsViewMac.

Don't convert the calls for acting as a drag-drop source, since that
will need further refactoring in the future.

Bug: 898608
Change-Id: Id85c2259daf74ecea3d145923b0a2f2974d182ed
Reviewed-on: https://chromium-review.googlesource.com/c/1416470
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@{#623769}
7 files changed