Revert "enable libaom"

This reverts commit 978bcb16.

Reason for revert: Broke linux msan builder:
https://ci.chromium.org/buildbot/chromium.memory/Linux%20MSan%20Builder/7457

Original change's description:
> enable libaom
> 
> Check out and build libaom by default.
> 
> AV1 playback is still guarded by a flag.
> 
> BUG=783519
> 
> Change-Id: Iac481ca03cc39b6241b3ffcf834689626c7e39f1
> Reviewed-on: https://chromium-review.googlesource.com/780581
> Commit-Queue: Johann Koenig <johannkoenig@google.com>
> Reviewed-by: Dale Curtis <dalecurtis@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#521207}

TBR=dalecurtis@chromium.org,johannkoenig@google.com

Change-Id: I6e86090b9d56c5ccf6b3a30bcc57248aee945661
No-Presubmit: true
No-Tree-Checks: true
No-Try: true
Bug: 783519
Reviewed-on: https://chromium-review.googlesource.com/804686Reviewed-by: Thomas Anderson <thomasanderson@chromium.org>
Commit-Queue: Thomas Anderson <thomasanderson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#521218}

DEPS

@@ -54,7 +54,7 @@ vars = {
   'checkout_telemetry_dependencies': False,
 
   # libaom provides support for AV1 but the bitstream is not frozen.
-  'checkout_libaom': True,
+  'checkout_libaom': False,
 
   # TODO(dpranke): change to != "small" once != is supported.
   'checkout_traffic_annotation_tools': 'checkout_configuration == "default"',