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There was a handy dandy testing version of NativeTheme in
view_unittest.cc that I'd like to reuse in upcoming unit tests for WebUI
Dark Mode support (https://crrev.com/c/1354685).

BUG=883049
R=sky@chromium.org

Change-Id: Ie39f6c42f5069450ec9933b1b83d4cb615ab713d
Reviewed-on: https://chromium-review.googlesource.com/c/1357607
Commit-Queue: Dan Beam <dbeam@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/master@{#613233}