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

chrome/common/page_load_metrics/page_load_timing.cc

Index: chrome/common/page_load_metrics/page_load_timing.cc
diff --git a/chrome/common/page_load_metrics/page_load_timing.cc b/chrome/common/page_load_metrics/page_load_timing.cc
index 3f0925893c40a0eba879b36cdb7bafa3130c783c..cbb311a645c3eadeeb503662009972495a508268 100644
--- a/chrome/common/page_load_metrics/page_load_timing.cc
+++ b/chrome/common/page_load_metrics/page_load_timing.cc
@@ -33,7 +33,9 @@ bool PageLoadTiming::operator==(const PageLoadTiming& other) const {
              other.parse_blocked_on_script_execution_duration &&
          parse_blocked_on_script_execution_from_document_write_duration ==
              other
-                 .parse_blocked_on_script_execution_from_document_write_duration;
+                 .parse_blocked_on_script_execution_from_document_write_duration &&
+         author_style_sheet_parse_duration_before_fcp ==
+             other.author_style_sheet_parse_duration_before_fcp;
 }
 
 bool PageLoadTiming::IsEmpty() const {
@@ -45,7 +47,8 @@ bool PageLoadTiming::IsEmpty() const {
          !parse_blocked_on_script_load_duration &&
          !parse_blocked_on_script_load_from_document_write_duration &&
          !parse_blocked_on_script_execution_duration &&
-         !parse_blocked_on_script_execution_from_document_write_duration;
+         !parse_blocked_on_script_execution_from_document_write_duration &&
+         !author_style_sheet_parse_duration_before_fcp;
 }
 
 PageLoadMetadata::PageLoadMetadata() {}