[Testing] Clean up CrossThreadStyleValueTest

This CL cleans up the CrossThreadStyleValueTest, which does proper
initialization for the WebThreadSupportingGC, and shut down the thread
at the end of the test, just like what we do in the

Bug: 895579
Change-Id: I7cf93fe5a192cd221a7f9490982d1a104110a232
Reviewed-on: https://chromium-review.googlesource.com/c/1427605
Reviewed-by: Anders Hartvoll Ruud <andruud@chromium.org>
Reviewed-by: Xida Chen <xidachen@chromium.org>
Commit-Queue: Xida Chen <xidachen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#625195}
1 file changed