[Mac] Use default caching for new tab button image

NSImage's default caching policy defers to the backing image representation.
In the new tab button's case, that's an NSCustomRep subclass, which has
a default caching policy of "always".

That means setting the image's policy SHOULD be a no-op, but in practice
it causes the linked bug*. This seems related to a known issue where the
new tab button won't redraw when it should (for example, setting high
contrast mode, crbug.com/774493). This change fixes the immediate bug while we 
continue to investigate the issue with redrawing

* Never mind that the image gets replaced! Mysteries abound.

Bug: 768327
Change-Id: I6873dd958a0197d8f4149f9a8e1b682a06c8498f
Reviewed-on: https://chromium-review.googlesource.com/716960
Commit-Queue: Leonard Grey <lgrey@chromium.org>
Reviewed-by: Elly Fong-Jones <ellyjones@chromium.org>
Cr-Commit-Position: refs/heads/master@{#508696}
1 file changed