Add a metrics extensions API.

See http://docs.google.com/View?id=dd4ngnpz_0dpsb8n96 for API proposal.

BUG=0
TEST=See unit tests as part of this CL

Review URL: http://codereview.chromium.org/657037

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@40130 0039d316-1c4b-4281-b951-d872f2087c98
diff --git a/chrome/chrome_browser.gypi b/chrome/chrome_browser.gypi
index b90d4ca1..0616843 100755
--- a/chrome/chrome_browser.gypi
+++ b/chrome/chrome_browser.gypi
@@ -880,6 +880,8 @@
         'browser/extensions/extension_install_ui.h',
         'browser/extensions/extension_message_service.cc',
         'browser/extensions/extension_message_service.h',
+        'browser/extensions/extension_metrics_module.cc',
+        'browser/extensions/extension_metrics_module.h',
         'browser/extensions/extension_page_actions_module.cc',
         'browser/extensions/extension_page_actions_module.h',
         'browser/extensions/extension_page_actions_module_constants.cc',