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..d0675047f60a876bebf270c84409bcf54c73a172 100644
--- a/components/metrics/proto/execution_context.proto
+++ b/components/metrics/proto/execution_context.proto
@@ -47,3 +47,23 @@ 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_START = 0;
+  MAIN_NAVIGATION_START = 1;
+  FIRST_NONEMPTY_PAINT = 2;
+  FIRST_CONTENTFUL_PAINT = 3;
+
+  SHUTDOWN_START = 4;
+}
+
+// Process activities. These are actions that start and end, likely many
+// times, throughout the lifetime of a process.
+enum ProcessActivity {
+  COLLECTING_METRICS = 0;
+  WRITING_PROFILES_STATE = 1;
+}

[Alexei Svitkine (slow), 2016/11/07 23:22:07]

I'm not super convinced of the utility of these, but happy to hear the use case.

Either we can chat about these offline or on this CL and continue the review
from there - or alternatively how about removing the activity stuff from this
initial CL and it can be landed as a follow-up CL later?

[bcwhite, 2016/11/08 01:02:36]

Yeah, we should talk face-to-face quickly.  It's basically there to record big
system activities that, if happen to be running concurrently to something under
test, could skew the samples.

If, to completely make up an example, you notice your code-under-test
occasionally spending a lot of time in utility method ABC, it might be nice to
know that activity XYZ (which also calls ABC) happened to also be running on
those occasions.

[Alexei Svitkine (slow), 2016/11/14 18:26:39]

Per offline discussion, we agreed to land the phases first and the activity can
be landed later if desired.

[bcwhite, 2016/11/16 18:27:44]

Done.