libbrillo: use base::Value instead of StringValue

StringValue is type aliased to base::Value now, so we should
move to explicit base::Value to avoid breaking when we uprev
libchrome.

BUG=b:37434548
TEST=unit tests

Change-Id: I77b4354ff63d71186c74deb5a5b0bc925a577653
Reviewed-on: https://chromium-review.googlesource.com/953648
Commit-Ready: Eric Caruso <ejcaruso@chromium.org>
Tested-by: Eric Caruso <ejcaruso@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
1 file changed