Make gfx::Image's NSImage constructor retain the incoming image.

The Mac convention for ObjC objects is that if a callee needs to
have a reference, it should take the reference itself. The
current behavior of gfx::Image's NSImage constructor is surprising
and relying on explaining it in the comment is inadequate.

Note that gfx::Image's UIImage constructor had a similar change
made to it in 2a1f622ddc372a0bd4206fb01c45f4ebbe37258b.

BUG=904645

Change-Id: I4dab00c3c7f3ff2f73e17520562d728b3a2e52aa
Reviewed-on: https://chromium-review.googlesource.com/c/1335957
Reviewed-by: Robert Sesek <rsesek@chromium.org>
Commit-Queue: Avi Drissman <avi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#608566}
9 files changed