ANGLE_DEFAULT_PLATFORM environment variable now accepts 'null' as
a valid option. This will set the default platform type to the
null backend.

Bug: angleproject:3893
Change-Id: Ieddcdf3ae840f927c1b276e56f3b753b67d9b73d
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1784062Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Mohan Maiya <m.maiya@samsung.com>
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>