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This was a stub implementation.  No one should actually rely on its
values, and returning null by default helps turn what could be subtle
runtime behavior bugs into crashes.

Provide the stub in ChromeViewsTestBase.  In production code, Widgets
should all be supplying a real theme provider.

Bug: none
Change-Id: I127048666ab3599f92f93a6d7627907344eae0a0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2045314
Commit-Queue: Peter Kasting <pkasting@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757978}