Remove base::StringValue

As a follow-up to typedeffing base::StringValue to base::Value in
http://crrev.com/2516363005 this change replaces all references to
base::StringValue by base::Value.

TBR=stevenjb@chromium.org
BUG=646113

Review-Url: https://codereview.chromium.org/2664753002
Cr-Commit-Position: refs/heads/master@{#454848}
492 files changed