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This adds configs for a manually-triggered trybot that runs release
blink_web_tests with --force-renderer-accessibility, in order to start
stabilizing serialization/deserialization code for all the content
tested by the entire web_tests suite.

See corresponding tools/build cl landed here: crrev.com/c/2454398

Bug: 1135251
Change-Id: Ic9db4673a3bf580225864f1aa6b1da6a6e682793
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2453446Reviewed-by: Stephen Martinis <martiniss@chromium.org>
Commit-Queue: Daniel Libby <dlibby@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#815296}