We had changed the configurations to run them in ES2 mode, while
ClearBuffer is an ES3-only method.

BUG=angle:769

Change-Id: I63d92af4702a133aa8bbf52444dfef59a4f602b0
Reviewed-on: https://chromium-review.googlesource.com/221064Reviewed-by: Brandon Jones <bajones@chromium.org>
Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>