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This CL adds support for EGL_ANGLE_external_context_and_surface.
With this extension, it wraps a native EGL context to an ANGLE EGL
context, so we can use ANGLE on a native EGL context. And call ANGLE
eglMakeCurrent() with this ANGLE context, will not change the native
EGL current context.
And if this external ANGLE context is created with
EGL_EXTERNAL_CONTEXT_SAVE_STATE_ANGLE. ANGLE will save GL state of
the native GL when this ANGLE context is made current. And ANGLE will
restore the saved GL state of the native context, when this ANGLE
context is released with eglMakeCurrent(EGL_NO_SURFACE, EGL_NO_CONTEXT).

Bug: 1157501
Change-Id: If7bb23dda18f09866fe33d185ef0f21a78a6ba1c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2638194
Commit-Queue: Peng Huang <penghuang@chromium.org>
Reviewed-by: Jonathan Backer <backer@chromium.org>
Auto-Submit: Peng Huang <penghuang@chromium.org>
Cr-Commit-Position: refs/heads/master@{#846194}