Simply exposing the extension string allows the passthrough command decoder to
initialize for testing.  It still fails all tests that use the extension.

BUG=angleproject:2722

Change-Id: I71f408d05009afb594835c0b741256e26209d93e
Reviewed-on: https://chromium-review.googlesource.com/1138824
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>