Add client_uuid to system profile

Add a new field called client_uuid to system profile to help simulate
field trial randomization on the server side.

Bug: 1058124
Change-Id: I098f9fe31f3d88a78d5e7d784c2edc498dc1e0b6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2085631
Commit-Queue: Weilun Shi <sweilun@chromium.org>
Commit-Queue: Alexei Svitkine <asvitkine@chromium.org>
Auto-Submit: Weilun Shi <sweilun@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Cr-Commit-Position: refs/heads/master@{#746537}

third_party/metrics_proto/README.chromium

 Name: Metrics Protos
 Short Name: metrics_proto
 URL: This is the canonical public repository
-Version: 297744444
-Date: 2020/02/28 UTC
+Version: 298662254
+Date: 2020/03/03 UTC
 License: BSD
 Security Critical: Yes
 

third_party/metrics_proto/system_profile.proto

@@ -15,7 +15,7 @@ import "extension_install.proto";
 
 // Stores information about the user's brower and system configuration.
 // The system configuration fields are recorded once per client session.
-// Next tag: 34
+// Next tag: 35
 message SystemProfileProto {
   // The time when the client was compiled/linked, in seconds since the epoch.
   optional int64 build_timestamp = 1;
@@ -757,6 +757,11 @@ message SystemProfileProto {
   // field trials.
   optional bool client_id_was_used_for_trial_assignment = 33;
 
+  // The unhashed client_id for this report. This is a uuid in its canonical
+  // textual represetnation in the form 8-4-4-4-12 for a total of 36 characters.
+  // Used to simulate field trial assignments for the client.
+  optional string client_uuid = 34;
+
   // Information about the A/V output device(s) (typically just a TV).
   // However, a configuration may have one or more intermediate A/V devices
   // between the source device and the TV (e.g. an A/V receiver, video