tracing: Use process start time as unique identifier

Use the process start time hashed with the process id as the unique
identifier for a process. This ensures that if there are two independent
copies of the Perfetto SDK in the same process (e.g., one in the app and
another in a system framework), events emitted by each will be
consistently interleaved on common thread and process tracks.

Bug: b/178314761
Change-Id: Icccfd6813da6ed8bf9a6d8fbf9c41d964b65b5fb
diff --git a/CHANGELOG b/CHANGELOG
index cc3e886..36fca95 100644
--- a/CHANGELOG
+++ b/CHANGELOG
@@ -5,6 +5,10 @@
     *
   UI:
     *
+  SDK:
+    * Use process start time hashed with the process id as a unique process
+      identifier, allowing multiple independent users of the SDK in the same
+      process to interleave their events on shared tracks.
 
 
 v12.1 - 2021-02-01: