This patch adds the support of overwriting angle_root to the one
defined outside ANGLE, which is useful for other projects which
intend to configure ANGLE in their GN builds.

Bug: angleproject:3263
Change-Id: I70a56a3d0afdca2eb66fb9619f0dd4d3efc58dee
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1522912
Commit-Queue: Jiawei Shao <jiawei.shao@intel.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>