First step to move common accessibility code out of content.

content/content_common.gypi

Index: content/content_common.gypi
diff --git a/content/content_common.gypi b/content/content_common.gypi
index a021c9418141ec96eaafc707623df4cd1a824ff8..d0c135fd7e3fb0cb7fd74a7c5437452b9b5e9f38 100644
--- a/content/content_common.gypi
+++ b/content/content_common.gypi
@@ -22,6 +22,14 @@
     '../base/base.gyp:base',
   ],
   'sources': [
+    'public/common/ax_node.cc',
+    'public/common/ax_node.h',
+    'public/common/ax_node_data.cc',
+    'public/common/ax_node_data.h',
+    'public/common/ax_tree.cc',
+    'public/common/ax_tree.h',
+    'public/common/ax_tree_update.cc',
+    'public/common/ax_tree_update.h',
     'public/common/bindings_policy.h',
     'public/common/child_process_host.h',
     'public/common/child_process_host_delegate.cc',
@@ -121,6 +129,13 @@
     'common/android/trace_event_binding.cc',
     'common/android/trace_event_binding.h',
     'common/appcache_messages.h',
+    'common/ax_node_impl.cc',
+    'common/ax_node_impl.h',
+    'common/ax_tree_impl.cc',
+    'common/ax_tree_impl.h',
+    'common/ax_tree_serializer.cc',
+    'common/ax_tree_serializer.h',
+    'common/ax_tree_source.h',
     'common/browser_plugin/browser_plugin_constants.cc',
     'common/browser_plugin/browser_plugin_constants.h',
     'common/browser_plugin/browser_plugin_messages.h',