This is a reland of e70f6aa6

Original change's description:
> ANGLE_platform_angle_device_context_volatile_* extensions
>
> 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>

Bug: angleproject:5104
Change-Id: I88265625a4bb4c1412532768d17d7b4356c7be41
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2508842Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: James Darpinian <jdarpinian@chromium.org>