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This change moves MergeDictionary() from base::DictionaryValue to the
parent class base::Value and relaxes its interface to accept base::Value
pointers as well. Appropriate CHECKs are added.

Bug: 646133
Change-Id: Ib6dab60ea2ff4f56b320e4a54f0782905aa3c4ae
Reviewed-on: https://chromium-review.googlesource.com/1158233Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jan Wilken Dörrie <jdoerrie@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579847}