Lacros: disable libaom for Lacros.

crrev.com/c/2617229 enabled libaom, and Lacros was the target, too.
However, it couldn't be built for the device.

TBR=jzern@chromium.org
BUG=1169028
TEST=Built lacros and ran on a device.

Change-Id: I0cfda8eddaa778cda72e1fa0b6965b4b84a996f2
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2642045Reviewed-by: James Cook <jamescook@chromium.org>
Commit-Queue: James Cook <jamescook@chromium.org>
Auto-Submit: Hidehiko Abe <hidehiko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#845683}

third_party/libaom/options.gni

@@ -3,7 +3,7 @@ import("//build/config/gclient_args.gni")
 
 declare_args() {
   # Enable encoding and decoding AV1 video files.
-  enable_libaom = !is_chromeos_ash && !is_android
+  enable_libaom = !is_chromeos_lacros && !is_chromeos_ash && !is_android
 
   # To be deprecated soon.
   enable_libaom_decoder = false