As part of the MANGLE work we want Chrome to be able to use ANGLE on OSX. Currently Chromium uses CGL and IOSurfaces to do the rendering so we need to make the same thing work through ANGLE.

There are two relevant ANGLE bugs:
 - [GL implementation for the GL back-end](https://bugs.chromium.org/p/angleproject/issues/detail?id=891) to provide GL contexts on Mac
 - [Make the Mac port work with CAOpenGLLayer](https://bugs.chromium.org/p/angleproject/issues/detail?id=1233) to provide EGL window surfaces for standalone applications on Mac. This will not be used by Chromium as it does its own thing with IOSurfaces.

The rough steps needed to make Chromium work with ANGLE on OSX are then:
 - Spec out and implement GL_TEXTURE_RECTANGLE in ANGLE
 - Spec out and implement an IOSurface version of [EGL_ANGLE_d3d_texture_client_buffer](https://chromium.googlesource.com/angle/angle/+/master/extensions/EGL_ANGLE_d3d_texture_client_buffer.txt)
 - Make Chromium use EGL with ANGLE's dylib on OSX, and have ui/gl/gl_image_io_surface.mm use the new ANGLE extension instead of CGLTexImageIOSurface2D.
 - Port workarounds, test, etc.
 - Profit.