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This patch adds an intermediate class to help with the document
transitions. The idea is that Blink would create this request, and
transfer it to the compositor on commit. When the compositor decides
to submit a frame to viz, it would then construct viz directives from
this request and send it over to viz.

This patch also adds an include and a type usage in Blink to ensure
presubmit is happy.

R=khushalsagar@chromium.org

Bug: 1150461
Change-Id: I8e2bcc18b8bd9f1de0d057a8bf608365ac5aa10b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2574600Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Reviewed-by: kylechar <kylechar@chromium.org>
Reviewed-by: Khushal <khushalsagar@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835340}