This instruments the plumbing for the extension without adding any
functionality or exposing the extensions string.

The extension text is also updated to reflect the new entry point
design and naming. Also this corrects a few mistakes.

This will be followed up by the tests (which won't run) and then
the extension functionality in ANGLE.

BUG=angleproject:1897

Change-Id: I5b009e23bc27da06b067375525bd6fc574027702