Split web_contents_view_mac into Cocoa and C++ files

Strictly cut-and-paste patch, should have no functional changes.

At present, we don't instantiate a WebContentsViewCocoa in the PWA
process (rather, we just create a vanilla NSView in
WebContentsNSViewBridge's constructor). To add support for drag-drop
in PWAs we'll use a WebContentsViewCocoa in the PWA process, and then
mojo-ify the boundaries needed for drag-drop support.

Bug: 898608
Change-Id: I90da19a0508872db8f39123d0e55d4bd24afe696
Reviewed-on: https://chromium-review.googlesource.com/c/1363966
Commit-Queue: ccameron <ccameron@chromium.org>
Reviewed-by: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614278}
6 files changed