[Chromecast] Set default ozone platform to "cast" on device builds.

All device Chromecast builds should default to "cast" platform. Desktop
builds will continue to use "headless" as default for now.

BUG=

Review URL: https://codereview.chromium.org/1519613002

Cr-Commit-Position: refs/heads/master@{#364472}

build/common.gypi

@@ -2386,10 +2386,18 @@
               ['disable_display==0', {
                 # Enable the Cast ozone platform on all A/V Cast builds.
                 'ozone_platform_cast%': 1,
+
+                # For desktop Chromecast builds, override the default "headless"
+                # platform with --ozone-platform=egltest
+                # TODO(slan|halliwell): Make the default platform "cast" on
+                # desktop too.
                 'conditions': [
                   ['OS=="linux" and target_arch!="arm"', {
                     'ozone_platform_egltest%': 1,
                     'ozone_platform_ozonex%': 1,
+                  }, {
+                    # On device builds, enable "cast" as the default platform.
+                    'ozone_platform%': 'cast',
                   }],
                 ],
               }],

ui/ozone/ozone.gni

@@ -35,13 +35,18 @@ declare_args() {
 
     if (is_chromecast) {
       if (!disable_display) {
+        # Enable the Cast ozone platform on all A/V Cast builds.
         ozone_platform_cast = true
 
-        # For desktop Chromecast builds, override the default "headless" platform
-        # with --ozone-platform=egltest
+        # For desktop Chromecast builds, override the default "headless"
+        # platform with --ozone-platform=egltest
+        # TODO(slan|halliwell): Make the default platform "cast" on desktop too.
         if (target_os == "linux" && target_cpu != "arm") {
           ozone_platform_egltest = true
           ozone_platform_ozonex = true
+        } else {
+          # On device builds, enable "cast" as the default platform.
+          ozone_platform = "cast"
         }
       }
     } else if (is_chromeos) {