Import execution_context.proto from Google3.

Bug=b:169624777

Change-Id: I765db98c825757d14ec8804298ea01fbd13fd176
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2577599Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Bin Du <dubin@google.com>
Cr-Commit-Position: refs/heads/master@{#834302}

third_party/metrics_proto/README.chromium

 Name: Metrics Protos
 Short Name: metrics_proto
 URL: This is the canonical public repository
-Version: 344139149
-Date: 2020/11/24 UTC
+Version: 346104568
+Date: 2020/12/07 UTC
 License: BSD
 Security Critical: Yes
 

third_party/metrics_proto/execution_context.proto

@@ -63,12 +63,24 @@ enum Thread {
 
   SERVICE_WORKER_THREAD = 15;
 
-  // Thread pool thread (in renderer process).
+  // DedicatedWorker thread in the renderer process.
+  DEDICATED_WORKER_THREAD = 18;
+
+  // Thread pool thread (can be in different Chrome processes).
   THREAD_POOL_THREAD = 16;
 
   // GPU memory thread (in renderer process).
   GPU_MEMORY_THREAD = 17;
 
+  // The thread for Chrome memory tracing (can be in different processes).
+  MEMORY_INFRA_THREAD = 19;
+
+  // Media thread (in renderer process).
+  MEDIA_THREAD = 20;
+
+  // WebRTC threads, e.g. WebRTC_Signaling,WebRTC_Network (in renderer process).
+  WEBRTC_THREAD = 21;
+
   // 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.