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

chrome/common/page_load_metrics/page_load_timing.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef CHROME_COMMON_PAGE_LOAD_METRICS_PAGE_LOAD_TIMING_H_
 #define CHROME_COMMON_PAGE_LOAD_METRICS_PAGE_LOAD_TIMING_H_
 
 #include "base/optional.h"
 #include "base/time/time.h"
 #include "third_party/WebKit/public/platform/WebLoadingBehaviorFlag.h"
@@ skipping 80 matching lines @@
 
   // Sum of times when the parser is executing a script that was inserted from
   // document.write. This duration must be less than or equal to
   // parse_blocked_on_script_load_duration. Note that this value may be updated
   // multiple times during the period between parse_start and parse_stop. Note
   // that some uncommon cases where scripts are loaded via document.write are
   // not currently covered by this field. See crbug/600711 for details.
   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().
 };
 
 struct PageLoadMetadata {
   PageLoadMetadata();
   bool operator==(const PageLoadMetadata& other) const;
   // These are packed blink::WebLoadingBehaviorFlag enums.
   int behavior_flags = blink::WebLoadingBehaviorNone;
 };
 
 }  // namespace page_load_metrics
 
 #endif  // CHROME_COMMON_PAGE_LOAD_METRICS_PAGE_LOAD_TIMING_H_