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This class contains GC pointers and should be traced. Due to
a bug in the GC plugin, this wasn't being enforced. This also
changes the class to be a part object to clarify lifetimes.

Bug: 724418
Change-Id: I46d037d7f5040bc942b6bdf54d8d8a134914134b
Reviewed-on: https://chromium-review.googlesource.com/509149Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#473763}