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This patch adds AggregatedFrame as a return value of
SurfaceAggregator::Aggregate. The reason for this is twofold:
1. It type checks the usage and ensures that we only forward the
   minimum set of things that we populate (one field for example
   was used although never initialized by anything except the ctor).
2. It allows us to separate CompositorFrame::render_pass_list from
   AggregatedFrame::render_pass_list with hopes of (in the future) type
   checking that RenderPassId post aggregation is remapped into a
   global space, whereas RenderPassId in CompositorFrame is local
   to the associated SurfaceId.

R=kylechar@chromium.org, weiliangc@chromium.org
Bug: 1106418

Change-Id: Ie6728d010c2f15171fc74616f064446df752f16f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2302710Reviewed-by: Sadrul Chowdhury <sadrul@chromium.org>
Reviewed-by: weiliangc <weiliangc@chromium.org>
Commit-Queue: vmpstr <vmpstr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789479}