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This change changes the various base::DictionaryValue::Set* methods to
return a pointer to the newly added Value. This is in response to the
discussion on https://codereview.chromium.org/2845113002/.

Furthermore, this is motivated by the fact that various insert methods
of std::map also return a handle to the just added element.

R=brettw@chromium.org
BUG=646113

Review-Url: https://codereview.chromium.org/2850773002
Cr-Commit-Position: refs/heads/master@{#468146}