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This provides a customization point for subclasses to override, to
change the Widget subclass that's instantiated by CreateTestWidget().
This will be necessary to remove DefaultThemeProvider, because
ChromeViewsTestBase will need to provide widgets with a non-null
ThemeProvider at that point, and subclassing Widget is the only way to
do so.

Bug: none
Change-Id: I9057301e2a86449832e036ce943d595e56954e74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2143518
Auto-Submit: Peter Kasting <pkasting@chromium.org>
Commit-Queue: Thomas Lukaszewicz <tluk@chromium.org>
Reviewed-by: Thomas Lukaszewicz <tluk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#757880}