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Also fix potential leaks by sticking with modern base::Value APIs and
unique_ptrs.

BUG=543015

Change-Id: I0f5ca7423b118faef43c51ada9db2a01a228c0dd
Reviewed-on: https://chromium-review.googlesource.com/691138Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Cr-Commit-Position: refs/heads/master@{#505209}