Renames the ANGLE_platform_angle_d3d extension to ANGLE_platform_angle
which acts as an umbrella extension for all the angle display types but
requires the sub-extensions (d3d, opengl, software) to be present for the
enums to be valid (similar to the OES_framebuffer_object extension).

BUG=angle:668

Change-Id: I2c00021558d8b781b20f948f5e0df03f734bd033
Reviewed-on: https://chromium-review.googlesource.com/202597Reviewed-by: Shannon Woods <shannonwoods@chromium.org>
Tested-by: Geoff Lang <geofflang@chromium.org>