Add initial support for EXT_disjoint_timer_query. 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/323450 Reviewed-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>