EXT_disjoint_timer_query is feature complete with the WebGL tests passing
with Chromium using ANGLE as a backend. There is some flakiness in the
timestamp query test on WebGL, but investigation revealed a bug on
Chromium's end and a fix is being made there. Since the extension is
feature complete, it is now enabled by default on OpenGL so that it can be
regression tested.

BUG=angleproject:1265

Change-Id: If018b7e3ae84aff7e40c73ff8e672a86689ae6c4
Reviewed-on: https://chromium-review.googlesource.com/324580
Tryjob-Request: Ian Ewell <ewell@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Ian Ewell <ewell@google.com>