libaom: temporarily disable

Disable libaom long enough to make it into a nightly build.
This will allow us to determine the binary impact on official
builds.

BUG=chromium:851691
TBR=tomfinegan@chromium.org
CC=dalecurtis@chromium.org

Change-Id: Id22aba8d9f59c93a930ca64efa238c3138b61fc1
Reviewed-on: https://chromium-review.googlesource.com/1113518Reviewed-by: Johann Koenig <johannkoenig@google.com>
Commit-Queue: Johann Koenig <johannkoenig@google.com>
Cr-Commit-Position: refs/heads/master@{#570045}

DEPS

@@ -71,7 +71,7 @@ vars = {
   'checkout_telemetry_dependencies': False,
 
   # libaom provides support for AV1 but the bitstream is not frozen.
-  'checkout_libaom': True,
+  'checkout_libaom': False,
 
   # By default do not check out the Oculus SDK. Only available for Googlers.
   'checkout_oculus_sdk' : 'checkout_src_internal and checkout_win',