This is useful for situations where angle is substituted for the system
library underneath an application oblivious to this change.  The
ANGLE_DEFAULT_PLATFORM variable is used when the display type is default
to control the default back-end.  Possible values are vulkan, gl and
d3d11.  If the value of this variable is not valid, or not supported by
the current platform, the usual defaults are applied.

Bug: angleproject:3125
Change-Id: I304b006e0a4149f78561038d01cede143640dafb
Reviewed-on: https://chromium-review.googlesource.com/c/1450211Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>