Looking at the EGL spec, it says for eglGetCurrentDisplay:

"The display for the current context in the calling thread, for the
current rendering API, is returned."

This implies that MakeCurrent binds a display to a Context. There's
also pretty clear language for the read/draw Surface as well, that
they can only be bound to one Context/thread at a time. Hence we
don't need to duplicate this storage in the egl::Thread structure,
merely storing a pointer to the current Context, which has access
to the read/draw Surface and current Display.

BUG=angleproject:1156

Change-Id: Ia3b99d50b3591012c43e851834c1af02ff62a33f
Reviewed-on: https://chromium-review.googlesource.com/538865Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>