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SurfaceAggregator::Aggregate() could attempt to use the last entry in
|dest_pass_list_| without verifying there was anything in it and crash.
There is an early exit for an empty list, but it's after the code uses
the last entry in the list.

The way that member variables are used during Aggregate() is a bit
awkward, many of them are used only for the duration of Aggregate() to
avoid passing variables around and need to be reset, while others need
to persist after Aggregate(). While it would be best if the temporary
state needed during Aggregate() was refactored out, this CL adds a
member function to reset variables needed only during Aggregate(). This
allows moving the early exit before using |dest_pass_list_|.

Bug: 1126897
Change-Id: Ie6979a637ba1f3f301394171ca74064fccc5c4f0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2404473Reviewed-by: weiliangc <weiliangc@chromium.org>
Commit-Queue: kylechar <kylechar@chromium.org>
Cr-Commit-Position: refs/heads/master@{#806322}