Have jni_generator.gypi specify include_dirs to dependent targets.

Rather than each dependent target of a target that generates jni headers having
to specify the appropriate include_dir manually, jni_generator.gypi can do it
via direct_dependent_settings.

TBR=darin@chromium.org

Review URL: https://codereview.chromium.org/111493006

git-svn-id: http://src.chromium.org/chrome/trunk/src/testing@241216 4ff67af0-8c30-449e-8e8b-ad334ec8d88c