Basic timer queries are supported and tested in the OpenGL backend
but are not enabled by default. A good portion of the existing query
code was also refactored for improved validation - specifically for
validating that the appropriate extensions are available.

BUG=angleproject:1265

Change-Id: Iebae994cd7a8d3ed3e9fc3776fe2f3d99caa9237
Reviewed-on: https://chromium-review.googlesource.com/323450Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tryjob-Request: Ian Ewell <ewell@google.com>
Tested-by: Ian Ewell <ewell@google.com>