Update uma protos from Google3.

Change-Id: If951059de8e60009df6a3f4b9075bdda56dd761e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1604042
Auto-Submit: Lakshman Annadorai <lakshmana@chromium.org>
Reviewed-by: Mike Wittman <wittman@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#659251}

third_party/metrics_proto/README.chromium

 Name: Metrics Protos
 Short Name: metrics_proto
 URL: This is the canonical public repository
-Version: 242980535
-Date: 2019/04/11 UTC
+Version: 247505755
+Date: 2019/05/09 UTC
 License: BSD
 Security Critical: Yes
 

third_party/metrics_proto/execution_context.proto

@@ -54,6 +54,11 @@ enum Thread {
   // Compositor thread (can be in both renderer and gpu processes).
   COMPOSITOR_THREAD = 11;
 
+  // Task scheduler thread.
+  SCHEDULER_WORKER_THREAD = 13;
+
+  COMPOSITOR_TILE_WORKER_THREAD = 14;
+
   // A Chrome thread not identified by any other enum. Defined for the benefit
   // of Chrome OS. Do not use for the Chrome sampling profiler; define a new
   // enum instead.

third_party/metrics_proto/ukm/source.proto

@@ -26,10 +26,18 @@ message Source {
   // An identifier for the source. This should be unique within a session.
   optional int64 id = 1;
 
-  // The previous source id for the tab this source is in. Should only
-  // be set for navigation sources. Note that only non-same-document
-  // sources are included here. If you want to include same-document
-  // navigations, use previous_same_document_source_id.
+  // The source id of the last committed non-same-document navigation for the
+  // tab this source is in. Should only be set for navigation sources. For
+  // backward compatibility, this field will never be set to the source id of a
+  // same-document source. In cases where the last committed navigation was a
+  // same-document navigation, previous_same_document_source_id will contain
+  // the source id of that previous same document navigation, while
+  // previous_source_id will contain the source id of the last committed
+  // non-same-document navigation. For example, if a user starts on page A (a
+  // non same document page load), then performs a same document navigation to
+  // B, then navigates to C (which could be either a same document or a non same
+  // document navigation), previous_source_id will reference A, and
+  // previous_same_document_source_id will reference B.
   optional int64 previous_source_id = 11;
 
   // The source id for the previous same document navigation, if the