Add CSSTiming to collect aggregate PLT-level stats about CSS.

chrome/common/page_load_metrics/page_load_timing.h

Index: chrome/common/page_load_metrics/page_load_timing.h
diff --git a/chrome/common/page_load_metrics/page_load_timing.h b/chrome/common/page_load_metrics/page_load_timing.h
index 223813514ef6aecbe679bfe634dae4eaaad72d3c..c59c3df27d7276222dd2cd4b4bbb84b01632669c 100644
--- a/chrome/common/page_load_metrics/page_load_timing.h
+++ b/chrome/common/page_load_metrics/page_load_timing.h
@@ -98,6 +98,9 @@ struct PageLoadTiming {
   base::Optional<base::TimeDelta>
       parse_blocked_on_script_execution_from_document_write_duration;
 
+  // Time spent parsing author style sheets.
+  base::Optional<base::TimeDelta> author_style_sheet_parse_duration_before_fcp;

[Bryan McQuade, 2016/11/04 14:11:29]

I think we're overdue for providing some amount of hierarchical structure here.
I should have defined a sub-struct that contained the parse-related timings, for
example. I'll go back and do that cleanup, but to start the process of providing
hierarchy and organization now, can you add a 'struct StyleSheetTiming' which
has one field for your new data point?

[Charlie Harrison, 2016/11/04 16:05:19]

Done.

+
   // If you add additional members, also be sure to update operator==,
   // page_load_metrics_messages.h, and IsEmpty().
 };