Update system_profile.proto from Google3.

Just contains a minor comment update.

Bug: b/123091364
Change-Id: If3f15b61fd50973cffb0d761e8f8e98171ee7b05
Tested: Just a comment change. Not needed.
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1552135
Auto-Submit: Jeremy Klein <jlklein@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Commit-Queue: Jeremy Klein <jlklein@chromium.org>
Cr-Commit-Position: refs/heads/master@{#649689}

third_party/metrics_proto/README.chromium

 Name: Metrics Protos
 Short Name: metrics_proto
 URL: This is the canonical public repository
-Version: 240367580
-Date: 2019/03/26 UTC
+Version: 241818494
+Date: 2019/04/03 UTC
 License: BSD
 Security Critical: Yes
 

third_party/metrics_proto/system_profile.proto

@@ -1019,7 +1019,8 @@ message SystemProfileProto {
   // Next Tag: 5
   message LinkedAndroidPhoneData {
     // The pii-free model name of the phone used for Better Together with this
-    // device. Will not be set if Better Together is not set up.
+    // device. Will not be set if Better Together is not set up. Hashed using
+    // variations::HashName() to produce a 32-bit SHA1 hash.
     optional fixed32 phone_model_name_hash = 1;
 
     // True if SmartLock is enabled on this Chromebook.