Upload UMA data using protocol buffers.

chrome/common/metrics/proto/chrome_user_metrics_extension.proto

Index: chrome/common/metrics/proto/chrome_user_metrics_extension.proto
diff --git a/chrome/common/metrics/proto/chrome_user_metrics_extension.proto b/chrome/common/metrics/proto/chrome_user_metrics_extension.proto
index 93902588f694f8e3294276551a271efe99da8303..f065cde5618c698501da66bacaf8a08686f76c3a 100644
--- a/chrome/common/metrics/proto/chrome_user_metrics_extension.proto
+++ b/chrome/common/metrics/proto/chrome_user_metrics_extension.proto
@@ -21,10 +21,10 @@ message ChromeUserMetricsExtension {
   // Technically, this id is unique to a top-level (one level above the
   // "Default" directory) Chrome user data directory [1], and so is shared among
   // all Chrome user profiles contained in this user data directory.
-  // A UserID of 0 is reserved for test data (monitoring and internal testing)
-  // and should normally be ignored in analysis of the data.
+  // An id of 0 is reserved for test data (monitoring and internal testing) and
+  // should normally be ignored in analysis of the data.
   // [1] http://www.chromium.org/user-experience/user-data-directory
-  optional fixed64 user_id = 1;
+  optional fixed64 client_id = 1;
 
   // The session id for this user.
   // Values such as tab ids are only meaningful within a particular session.