Allow overwriting angle_root in non-Chromium GN configurations

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>

build_overrides/angle.gni

@@ -2,6 +2,9 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
+# Override for angle_root
+angle_root = "//"
+
 # Overrides for ANGLE's dependencies
 angle_glslang_dir = "//third_party/glslang/src"
 angle_googletest_dir = "//third_party/googletest/src"

gni/angle.gni

@@ -13,12 +13,8 @@ if (is_android) {
 }
 
 if (build_with_chromium) {
-  angle_root = "//third_party/angle"
-
   import("//ui/ozone/ozone.gni")
 } else {
-  angle_root = "//"
-
   declare_args() {
     ozone_platform_gbm = false
   }