Add process lifetime annotations to stack samples.

components/metrics/proto/execution_context.proto

Index: components/metrics/proto/execution_context.proto
diff --git a/components/metrics/proto/execution_context.proto b/components/metrics/proto/execution_context.proto
index 3d8679fe9db2bd8ce69660940c35c7f2416e434a..9076a3d62f39acdd5c0bcbae2ef1b1888d2f9cda 100644
--- a/components/metrics/proto/execution_context.proto
+++ b/components/metrics/proto/execution_context.proto
@@ -47,3 +47,18 @@ enum Thread {
   RENDER_THREAD = 9;
   UTILITY_THREAD = 10;
 }
+
+// Process phases, or where in the lifetime of the process it is such as
+// startup, normal operation, shutdown, etc. These don't necessarily occur in
+// the order defined here so it's fine to add new ones to further segregrate
+// the lifetime of a process.
+enum ProcessPhase {
+  MAIN_LOOP_STARTED = 0;
+  FIRST_NON_EMPTY_PAINT = 1;
+}
+
+// Process activities. These are actions that start and end, likely many
+// times, throughout the lifetime of a process.
+enum ProcessActivity {
+  METRICS_COLLECTION = 0;
+}