Chromium has a jsoncpp mirror that matches github and has a ninja build
config so use that instead of Android mirror.

Bug: angleproject:2794
Change-Id: I48423818817f439f3a43f5edee557deae24bf028
Reviewed-on: https://chromium-review.googlesource.com/1203940Reviewed-by: Ian Elliott <ianelliott@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Ian Elliott <ianelliott@google.com>