Build API: Add CHROME_ORIGIN env var when chrome root set.

BUG=chromium:950006
TEST=./run_tests

Change-Id: I976ef362b7abf163cd8fbd324bfa3a3f34c30704
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/1815067
Commit-Queue: Alex Klein <saklein@chromium.org>
Commit-Queue: David Burger <dburger@chromium.org>
Tested-by: Alex Klein <saklein@chromium.org>
Tested-by: David Burger <dburger@chromium.org>
Reviewed-by: David Burger <dburger@chromium.org>
diff --git a/api/controller/controller_util.py b/api/controller/controller_util.py
index 861e452..fa22fbf 100644
--- a/api/controller/controller_util.py
+++ b/api/controller/controller_util.py
@@ -47,6 +47,10 @@
   if use_flags:
     env['USE'] = ' '.join(use_flags)
 
+  # Make sure it'll use the local source to build chrome when we have it.
+  if chrome_root:
+    env['CHROME_ORIGIN'] = 'LOCAL_SOURCE'
+
   # TODO(saklein) Remove the default when fully integrated in recipes.
   env['FEATURES'] = 'separatedebug'
   if features: