Change from Kimmo Kinnunen downstream:
https://bugs.webkit.org/show_bug.cgi?id=216106

Add two extensions for EAGL and CGL backends to declare the
underlying platform context being "volatile". It means that
the thread-global current context is being modified behind
ANGLE. If ANGLE context is marked volatile for a particular
API, it will sync the underlying context for every EGL
function that needs the context. Most intuitive use is
for the client to call eglMakeCurrent before calling any
gl function if the client knowns the platform state might
be dirty.

Implement eglReleaseThread for EAGL and CGL backends.
Releasing thread will unset the platform current context.

Fix a bug of omitting EGL_ANGLE_device_eagl from being
advertised.

Bug: angleproject:5104
Change-Id: I1ec98ad35bc0caada23556ae8697fdef20f65b1a
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2486548
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Reviewed-by: Kenneth Russell <kbr@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>