Add the extension text, entry points and validations.

BUG=chromium:849576
TEST=angle_end2end_tests

Change-Id: I4c06ee30e4f4fe9bb1c1fecada747b9c78fed0ea
Reviewed-on: https://chromium-review.googlesource.com/1103789Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jie A Chen <jie.a.chen@intel.com>