Provide skeleton for ChromeVox next.

build/common.gypi

Index: build/common.gypi
diff --git a/build/common.gypi b/build/common.gypi
index def3eaf76e23551abde55b878fbd2ab7ab6703de..721a07ae4182e8a60d22616b454e2a25a397a8da 100644
--- a/build/common.gypi
+++ b/build/common.gypi
@@ -902,6 +902,9 @@
       # for the HRTF panner in WebAudio.
       'use_concatenated_impulse_responses': 1,
 
+      # Whether to use the upcoming version of ChromeVox.
+      'use_chromevox_next%': 0,
+
       # You can set the variable 'use_official_google_api_keys' to 1
       # to use the Google-internal file containing official API keys
       # for Google Chrome even in a developer build.  Setting this
@@ -1370,6 +1373,9 @@
     # Support ChromeOS touchpad gestures with ozone.
     'use_evdev_gestures%': 0,
 
+    # Whether to use the upcoming version of ChromeVox.
+    'use_chromevox_next%': '<(use_chromevox_next)',
+
     'conditions': [
       # Enable the Syzygy optimization step for the official builds.
       ['OS=="win" and buildtype=="Official" and syzyasan!=1', {
@@ -1955,6 +1961,10 @@
         'grit_defines': ['-D', 'enable_service_discovery'],
         'enable_service_discovery%': 1
       }],
+      ['use_chromevox_next==1', {
+        'grit_defines': ['-D', 'use_chromevox_next'],
+      }],
+
       ['clang_use_chrome_plugins==1 and OS!="win"', {
         'clang_chrome_plugins_flags': [
           '<!@(<(DEPTH)/tools/clang/scripts/plugin_flags.sh)'