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This patch adds a transition directive class to viz. It isn't currently
used outside of unittests, but will be referenced in cc and indirectly
from Blink in order to construct the right directive for viz to execute.

R=kylechar@chromium.org

Bug: 1150461
Change-Id: I9cbe63507148405b0a363ee6d957d76b1a18b34e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2568985
Commit-Queue: vmpstr <vmpstr@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#833759}