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

chrome/common/page_load_metrics/page_load_metrics_messages.h

Index: chrome/common/page_load_metrics/page_load_metrics_messages.h
diff --git a/chrome/common/page_load_metrics/page_load_metrics_messages.h b/chrome/common/page_load_metrics/page_load_metrics_messages.h
index 4db7ec3e902543566fef0c95405490c1790dcbd0..7d2c67332b44364d27329d2caeae527f5e41dccf 100644
--- a/chrome/common/page_load_metrics/page_load_metrics_messages.h
+++ b/chrome/common/page_load_metrics/page_load_metrics_messages.h
@@ -12,6 +12,11 @@
 #define IPC_MESSAGE_START PageLoadMetricsMsgStart
 
 // See comments in page_load_timing.h for details on each field.
+
+IPC_STRUCT_TRAITS_BEGIN(page_load_metrics::StyleSheetTiming)
+  IPC_STRUCT_TRAITS_MEMBER(author_style_sheet_parse_duration_before_fcp)
+IPC_STRUCT_TRAITS_END()
+
 IPC_STRUCT_TRAITS_BEGIN(page_load_metrics::PageLoadTiming)
   IPC_STRUCT_TRAITS_MEMBER(navigation_start)
   IPC_STRUCT_TRAITS_MEMBER(response_start)
@@ -31,6 +36,7 @@ IPC_STRUCT_TRAITS_BEGIN(page_load_metrics::PageLoadTiming)
   IPC_STRUCT_TRAITS_MEMBER(parse_blocked_on_script_execution_duration)
   IPC_STRUCT_TRAITS_MEMBER(
       parse_blocked_on_script_execution_from_document_write_duration)
+  IPC_STRUCT_TRAITS_MEMBER(style_sheet_timing)
 IPC_STRUCT_TRAITS_END()
 
 IPC_STRUCT_TRAITS_BEGIN(page_load_metrics::PageLoadMetadata)