Support create ANGLE EGLContext from an external EGLContext 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>