Add Dawn got_revision_mapping to glient config

BUG=875245

Change-Id: I50425aef624d00c1aa6fd7c18244fea128c15842
Reviewed-on: https://chromium-review.googlesource.com/1209643
Commit-Queue: Andrii Shyshkalov <tandrii@chromium.org>
Reviewed-by: Andrii Shyshkalov <tandrii@chromium.org>

recipes/recipe_modules/gclient/config.py

@@ -418,3 +418,4 @@ def dawn(c):
   soln = c.solutions.add()
   soln.name = 'dawn'
   soln.url = 'https://dawn.googlesource.com/dawn.git'
+  c.got_revision_mapping['dawn'] = 'got_revision'