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

build/common.gypi

 # Copyright (c) 2012 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # IMPORTANT:
 # Please don't directly include this file if you are building via gyp_chromium,
 # since gyp_chromium is automatically forcing its inclusion.
 {
   # Variables expected to be overriden on the GYP command line (-D) or by
   # ~/.gyp/include.gypi.
@@ skipping 2369 matching lines @@
       ['use_ozone==1 and ozone_auto_platforms==1', {
         # Use headless as the default platform.
         'ozone_platform%': 'headless',
         'ozone_platform_headless%': 1,
         'conditions': [
           ['chromecast==1', {
             'conditions': [
               ['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',
                   }],
                 ],
               }],
             ],
           }, {  # chromecast!=1
             'conditions': [
               ['OS=="chromeos"', {
                 'ozone_platform_gbm%': 1,
                 'ozone_platform_egltest%': 1,
               }, {
@@ skipping 3994 matching lines @@
     # settings in target dicts.  SYMROOT is a special case, because many other
     # Xcode variables depend on it, including variables such as
     # PROJECT_DERIVED_FILE_DIR.  When a source group corresponding to something
     # like PROJECT_DERIVED_FILE_DIR is added to a project, in order for the
     # files to appear (when present) in the UI as actual files and not red
     # red "missing file" proxies, the correct path to PROJECT_DERIVED_FILE_DIR,
     # and therefore SYMROOT, needs to be set at the project level.
     'SYMROOT': '<(DEPTH)/xcodebuild',
   },
 }