Instead of having separate GetGlobal* functions, interact with the global
objects through a single Thread object.  This reduces the number of TLS
lookups in many EGL functions and allows the Thread object to be passed
down to other objects if needed.

BUG=angleproject:1618

Change-Id: I1f9a89e8899d637633f4e91fda0e38ac308dd020
Reviewed-on: https://chromium-review.googlesource.com/409637Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Geoff Lang <geofflang@chromium.org>