Revert "Revert "enable libaom""

This reverts commit f7eb8650.

The msan build failure has been addressed:
https://chromium-review.googlesource.com/c/chromium/src/+/806634

Original change's description:
> Reason for revert: Broke linux msan builder:
> https://ci.chromium.org/buildbot/chromium.memory/Linux%20MSan%20Builder/7457

BUG=783519

Cq-Include-Trybots: master.tryserver.chromium.linux:linux_chromium_msan_rel_ng

R=tomfinegan@chromium.org,dalecurtis@chromium.org

Change-Id: I720447e099700e1bce0868590d31943beb370643
Reviewed-on: https://chromium-review.googlesource.com/806875Reviewed-by: Tom Finegan <tomfinegan@chromium.org>
Commit-Queue: Johann Koenig <johannkoenig@google.com>
Cr-Commit-Position: refs/heads/master@{#521477}

DEPS

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