Add angle_root custom var to ANGLE gclient config.

This should allow for any recipe that uses the ANGLE repo to run
gclient sync with the right folder structure. Currently gclient sync
will check out third_party directories to the wrong folder.

Bug: angleproject:3288
Change-Id: Ia97592fdc0c9f3ec70c2d4a05d9934a6bc996507
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1551019
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>

recipes/recipe_modules/gclient/config.py

@@ -411,6 +411,12 @@ def angle(c):
   soln = c.solutions.add()
   soln.name = 'angle'
   soln.url = 'https://chromium.googlesource.com/angle/angle.git'
+  # Standalone developer angle builds want the angle checkout in the same
+  # directory the .gclient file is in.  Bots want it in a directory called
+  # 'angle'.  To make both cases work, the angle DEPS file pulls deps and runs
+  # hooks relative to the variable "root" which is set to . by default and
+  # then to 'angle' in the recipes here:
+  soln.custom_vars = {'angle_root': 'angle'}
   c.got_revision_mapping['angle'] = 'got_revision'
 
 @config_ctx()