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Implementing the intant theme test in a non-flaky way. Instead of using
OnThemeChange event to determine whether the theme has applied on existing
tab/current tab/new tab, we directly check the css setting is equal to the
new theme or not.

Bug: 335297
Change-Id: Ic366e180281b23ccdffcbcd9e4b2144485591607
Reviewed-on: https://chromium-review.googlesource.com/1208939
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Kristi Park <kristipark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#589229}