Set the use_cras GN arg to true when building for CrOS.

This arg never changes among all supported CrOS device builds of Chrome.
By giving it a proper default val in those builds, we can reduce the amount
of inputs needed when building for CrOS (ie: simple chrome & chromeos-chrome
ebuild).

Bug: 937821
Change-Id: If383391938d84d4f584854ceb7c44428c3aa094b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1962796Reviewed-by: Xiaohan Wang <xhwang@chromium.org>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#724078}

media/media_options.gni

@@ -4,6 +4,7 @@
 
 import("//build/config/chrome_build.gni")
 import("//build/config/chromecast_build.gni")
+import("//build/config/chromeos/args.gni")
 import("//build/config/features.gni")
 import("//build/config/jumbo.gni")
 import("//media/gpu/args.gni")
@@ -58,7 +59,7 @@ declare_args() {
   }
 
   # Override to dynamically link the cras (ChromeOS audio) library.
-  use_cras = false
+  use_cras = is_chromeos_device
 
   # Enables AC3/EAC3 audio demuxing. This is enabled only on Chromecast, since
   # it only provides demuxing, and is only useful for AC3/EAC3 audio