This allows an application to use EGLDeviceEXT to initialize EGL.

For example, if an application wants to initialize EGL using an existing D3D11
device (instead of ANGLE creating its D3D device), then the app may create an
EGLDeviceEXT using EGL_ANGLE_device_creation_d3d11, and use this device to
initialize EGL via EGL_EXT_platform_device.

BUG=angleproject:1190

Change-Id: Ife91ce95a63e29eb2b3f05aedfb668e4cac8f5ce
Reviewed-on: https://chromium-review.googlesource.com/313444Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Austin Kinross <aukinros@microsoft.com>