For Android WebView, Android creates an EGLContext, EGLSurface
and FBO, and makeCurrent on them, and then calls WebView draw
function to draw the WebView content on the current EGLSurface or
binded FBO. So to use ANGLE in WebView, this CL adds a way to
create ANGLE EGLContext from an external EGLContext, and save
and restore GL state in eglMakeCurrent().

Bug: angleproject:5509
Change-Id: I874986813117f125e23e975ea1adc51ac5b3a631
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2615239
Commit-Queue: Peng Huang <penghuang@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jonah Ryan-Davis <jonahr@google.com>