MacViews: Get c/b/ui/views/tabs to build on Mac

Isolate aura and ash dependencies since they are not included on Mac.
Change GetLocalProcessWindowAtPoint to work with gfx::NativeWindow.
Add window_finder_mac.mm

BUG=425229

Review URL: https://codereview.chromium.org/669243002

Cr-Commit-Position: refs/heads/master@{#301235}
8 files changed