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This CL adds checkbox to animation tests inside WebView system shell
that enables external stencil by adding rounded corners clip path to
WebView.

Bug: 956709

Change-Id: Ia918bf2fa6af522bdda90d559eddf24c48172dbc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2075970
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Reviewed-by: Bo <boliu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#745068}