Split UMA metric for Style.UpdateTime.

Measure style recalc and layout tree building separately. We now don't
create samples for UpdateStyle() calls where both style and layout trees
are clean.

Bug: 851782
Change-Id: Icfa94f5a231775cbd9ea0aa80b8b693502b3ca42
Reviewed-on: https://chromium-review.googlesource.com/1156512Reviewed-by: Steven Holte <holte@chromium.org>
Commit-Queue: Rune Lillesveen <futhark@chromium.org>
Cr-Commit-Position: refs/heads/master@{#579731}

third_party/blink/renderer/core/dom/document.cc

@@ -2242,7 +2242,6 @@ void Document::UpdateStyle() {
   TRACE_EVENT_BEGIN0("blink,blink_style", "Document::updateStyle");
   RUNTIME_CALL_TIMER_SCOPE(V8PerIsolateData::MainThreadIsolate(),
                            RuntimeCallStats::CounterId::kUpdateStyle);
-  TimeTicks start_time = CurrentTimeTicks();
 
   unsigned initial_element_count = GetStyleEngine().StyleForElementCount();
 
@@ -2283,6 +2282,7 @@ void Document::UpdateStyle() {
   if (Element* document_element = documentElement()) {
     if (document_element->ShouldCallRecalcStyle(change)) {
       TRACE_EVENT0("blink,blink_style", "Document::recalcStyle");
+      SCOPED_BLINK_UMA_HISTOGRAM_TIMER_HIGHRES("Style.RecalcTime");
       Element* viewport_defining = ViewportDefiningElement();
       ReattachLegacyLayoutObjectList legacy_layout_objects(*this);
       legacy_layout_objects.WillRecalcStyle();
@@ -2296,6 +2296,7 @@ void Document::UpdateStyle() {
     if (document_element->NeedsReattachLayoutTree() ||
         document_element->ChildNeedsReattachLayoutTree()) {
       TRACE_EVENT0("blink,blink_style", "Document::rebuildLayoutTree");
+      SCOPED_BLINK_UMA_HISTOGRAM_TIMER_HIGHRES("Style.RebuildLayoutTreeTime");
       ReattachLegacyLayoutObjectList legacy_layout_objects(*this);
       WhitespaceAttacher whitespace_attacher;
       document_element->RebuildLayoutTree(whitespace_attacher);
@@ -2327,10 +2328,6 @@ void Document::UpdateStyle() {
         "blink,blink_style", "Document::updateStyle", "resolverAccessCount",
         GetStyleEngine().StyleForElementCount() - initial_element_count);
   }
-
-  DEFINE_STATIC_LOCAL(CustomCountHistogram, update_histogram,
-                      ("Style.UpdateTime", 0, 10000000, 50));
-  update_histogram.CountMicroseconds(CurrentTimeTicks() - start_time);
 }
 
 void Document::ViewportDefiningElementDidChange() {

tools/metrics/histograms/histograms.xml

@@ -98684,6 +98684,20 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="Style.RebuildLayoutTreeTime" units="microseconds">
+  <owner>futhark@chromium.org</owner>
+  <summary>
+    Microseconds spent in RebuildLayoutTree called from Document::UpdateStyle.
+  </summary>
+</histogram>
+
+<histogram name="Style.RecalcTime" units="microseconds">
+  <owner>futhark@chromium.org</owner>
+  <summary>
+    Microseconds spent in RecalcStyle called from Document::UpdateStyle.
+  </summary>
+</histogram>
+
 <histogram name="Style.TotalLazyRules" units="count">
   <obsolete>
     Deprecated 07/2018, no longer used.
@@ -98707,6 +98721,10 @@ uploading your change for review.
 </histogram>
 
 <histogram name="Style.UpdateTime" units="microseconds">
+  <obsolete>
+    Deprecated 07/2018, replaced by Style.RecalcTime and
+    Style.RebuildLayoutTreeTime.
+  </obsolete>
   <owner>futhark@chromium.org</owner>
   <summary>
     Microseconds spent in Document::UpdateStyle.