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Add interfaces and functions to allow passing in and compositing
contents from multiple frames in pdf compositor service.

Each frame is identified by its global unique id (rph id and rfh id).
Content id is a unique id within its process. Each frame may have oop
subframes. The pdf compositor service will keep track of all the frame
contents and their relationship. When all subframes are painted, the
service can composite the entire frame and convert it into PDF format.

Please refer document goo.gl/ACV6VM for more explanations.

BUG=455764

Change-Id: I3213d9ae0ca85d0ea695dd13acb00938499aa73b
Reviewed-on: https://chromium-review.googlesource.com/767912Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Wei Li <weili@chromium.org>
Cr-Commit-Position: refs/heads/master@{#531687}