Add flag to ANGLE build overrides.

This will unblock supporting a build variant in ANGLE where we
don't have Chromium supporting folders. This will enable us to
port to the Skia standalone build.

Bug: angleproject:3943
Change-Id: Ieb88f18b34d330458071c047f03a1c1d420b3d25
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1834909Reviewed-by: Nico Weber <thakis@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Cr-Commit-Position: refs/heads/master@{#702200}

build_overrides/angle.gni

@@ -5,6 +5,9 @@
 # Override for angle_root
 angle_root = "//third_party/angle"
 
+# True if ANGLE can access build/, testing/ and other Chrome folders.
+angle_has_build = true
+
 # Overrides for ANGLE's dependencies
 angle_glslang_dir = "//third_party/glslang/src"
 angle_googletest_dir = "//third_party/googletest/src"