Include scheduling-only threads in tracks

tracing/trace_viewer.gypi

Index: tracing/trace_viewer.gypi
diff --git a/tracing/trace_viewer.gypi b/tracing/trace_viewer.gypi
index 73d186392e898caae7cfc0a5ffb082d7587a9e69..b7e404778f9dc97b75e65fa295a9e64df9acc544 100644
--- a/tracing/trace_viewer.gypi
+++ b/tracing/trace_viewer.gypi
@@ -485,6 +485,7 @@
       'tracing/ui/tracks/rect_track.html',
       'tracing/ui/tracks/ruler_track.html',
       'tracing/ui/tracks/sample_track.html',
+      'tracing/ui/tracks/sched_group_track.html',

[charliea (OOO until 10-5), 2016/10/26 21:23:37]

I'm not sure that sched_group_track conveys the right idea here.

All of the existing *_group_tracks
(https://cs.corp.google.com/github/catapult-project/catapult/tracing/tracing/u...)
help display entities called ______Group (e.g. ObjectInstanceGroup, SliceGroup,
AsyncSliceGroup). In this case, we're instead displaying a group of threads that
have nowhere else to live.

Another problem is that this leaks some implementation details of ftrace into
the view code: most people don't know what a sched is, or what a group of
sched's might look like. 

I think that maybe something like other_thread_track.html might be a better
name, although I still don't love it: I just can't think of anything better.
Then, in that file, we describe that this is a place for "other" threads to live
that don't have much information about them.

[John Reck, 2016/10/26 21:55:47]

Done.

       'tracing/ui/tracks/slice_group_track.html',
       'tracing/ui/tracks/slice_track.html',
       'tracing/ui/tracks/spacing_track.html',