libweave: 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: Ic088520ea2a5fe103ccdfc7e1508798483bf3b48
Reviewed-on: https://chromium-review.googlesource.com/956404
Commit-Ready: Eric Caruso <ejcaruso@chromium.org>
Tested-by: Eric Caruso <ejcaruso@chromium.org>
Reviewed-by: Dan Erat <derat@chromium.org>
6 files changed