Upload UMA data using protocol buffers.

chrome/common/metrics/proto/system_profile.proto

Index: chrome/common/metrics/proto/system_profile.proto
diff --git a/chrome/common/metrics/proto/system_profile.proto b/chrome/common/metrics/proto/system_profile.proto
index 3a481a41b75747d97ee5d8cb860c1148e39f65ef..9923af841375c208db0aa57b8c4b589a530712e5 100644
--- a/chrome/common/metrics/proto/system_profile.proto
+++ b/chrome/common/metrics/proto/system_profile.proto
@@ -11,7 +11,7 @@ option optimize_for = LITE_RUNTIME;
 
 package metrics;
 
-// Next tag: 9
+// Next tag: 10
 message SystemProfileProto {
   // The time when the client was compiled/linked, in seconds since the epoch.
   optional int64 build_timestamp = 1;
@@ -226,4 +226,19 @@ message SystemProfileProto {
     repeated PluginStability plugin_stability = 22;
   }
   optional Stability stability = 8;
+
+  // Description of a field trial or experiment that the user is currently
+  // enrolled in.
+  // All metrics reported in this upload can potentially be influenced by the
+  // field trial.
+  message FieldTrial {
+    // The name of the field trial, as a 32-bit identifier.
+    // Currently, the identifier is a hash of the field trial's name.
+    optional fixed32 name_id = 1;
+
+    // The user's group within the field trial, as a 32-bit identifier.
+    // Currently, the identifier is a hash of the group's name.
+    optional fixed32 group_id = 2;
+  }
+  repeated FieldTrial field_trial = 9;
 }