Build ChromeVox using gyp.

This adds tools and gyp targets to build ChromeVox directly from its original
source in the Chromium repository instead of using precompiled obfuscated
JavaScript binaries.  In addition, this adds infrastructure to run
ChromeVox 'unbundled' inside of a chromeos build.

Note that this is still experimental and not enabled by default.  Also, the
manifest to run ChromeVox unbundled is not provided yet.

BUG=371692
R=dtseng@chromium.org,dmazzoni@chromium.org

Review URL: https://codereview.chromium.org/299703003

git-svn-id: http://src.chromium.org/svn/trunk/src/build@272146 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

common.gypi

@@ -1388,6 +1388,11 @@
     'ozone_platform_ozonex%': 0,
     'ozone_platform_test%': 0,
 
+    # Chrome OS: whether to build ChromeVox from sources in the Chromium
+    # repository rather than using precompiled JavaScript in
+    # chrome/third_party/chromevox.  This is still experimental.
+    'use_migrated_chromevox%': 0,
+
     # Chrome OS: whether to also build the upcoming version of
     # ChromeVox, which can then be enabled via a command-line switch.
     'use_chromevox_next%': 0,