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This is a reland of 7df6fead, which was
speculatively reverted as possible cause of crbug.com/788192, since the
CL originally landed only after that regression.

Original change's description:
> Migrate call-sites away from un-managed PlatformHandle[Vector]s.
>
> As the first step to merging ScopedPlatformHandle into PlatformHandle
> to have a single, always-managed container for platform handles, we:
>
> - Replace ScopedPlatformHandleVectorPtr and PlatformHandleVector
>   with vector<ScopedPlatformHandle>.
> - Replace PlatformHandle with ScopedPlatformHandle in some core APIs.
> - Use const& to pass handles to APIs without passing ownership, for
>   both individual platform handles, and vectors.
>
> Bug: 753541
> Change-Id: Id965dcf3770ef11bf36f78277fa7199ea779599e
> Reviewed-on: https://chromium-review.googlesource.com/742309
> Reviewed-by: Ricky Liang <jcliang@chromium.org>
> Reviewed-by: Ken Rockot <rockot@chromium.org>
> Reviewed-by: Dominick Ng <dominickn@chromium.org>
> Reviewed-by: Luis Hector Chavez <lhchavez@chromium.org>
> Commit-Queue: Wez <wez@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#518343}

Bug: 753541
TBR: lhchavez, jcliang, dominickn, rockot
Change-Id: I9d50f06052b2118aec8872ca149504fbed245f1a
Reviewed-on: https://chromium-review.googlesource.com/792255Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#519505}