Add UMA histogram DomDistiller.Time.DistillPage

This measures time spent in DistillPageImpl(), which is mostly
ExecuteJavaScript(). This includes JavaScript compilation and execution.

We already have DomDistiller.Time.DistillationTotal, but it is measured
within JavaScript.

Review URL: https://codereview.chromium.org/1131493005

Cr-Commit-Position: refs/heads/master@{#329472}

components/dom_distiller/core/distiller_page.cc

@@ -88,6 +88,7 @@ void DistillerPage::DistillPage(
   // the callback to OnDistillationDone happens.
   ready_ = false;
   distiller_page_callback_ = callback;
+  distillation_start_ = base::TimeTicks::Now();
   DistillPageImpl(gurl, GetDistillerScriptWithOptions(options,
                                                       StringifyOutput(),
                                                       CreateNewContext()));
@@ -110,6 +111,11 @@ void DistillerPage::OnDistillationDone(const GURL& page_url,
     if (!found_content) {
       DVLOG(1) << "Unable to parse DomDistillerResult.";
     } else {
+      base::TimeDelta distillation_time =
+          base::TimeTicks::Now() - distillation_start_;
+      UMA_HISTOGRAM_TIMES("DomDistiller.Time.DistillPage", distillation_time);
+      VLOG(1) << "DomDistiller.Time.DistillPage = " << distillation_time;
+
       if (distiller_result->has_timing_info()) {
         const dom_distiller::proto::TimingInfo& timing =
             distiller_result->timing_info();
@@ -139,6 +145,8 @@ void DistillerPage::OnDistillationDone(const GURL& page_url,
           UMA_HISTOGRAM_TIMES(
               "DomDistiller.Time.DistillationTotal",
               base::TimeDelta::FromMillisecondsD(timing.total_time()));
+          VLOG(1) << "DomDistiller.Time.DistillationTotal = " <<
+              base::TimeDelta::FromMillisecondsD(timing.total_time());
         }
       }
       if (distiller_result->has_statistics_info()) {

components/dom_distiller/core/distiller_page.h

@@ -66,6 +66,7 @@ class DistillerPage {
  private:
   bool ready_;
   DistillerPageCallback distiller_page_callback_;
+  base::TimeTicks distillation_start_;
   DISALLOW_COPY_AND_ASSIGN(DistillerPage);
 };
 

tools/metrics/histograms/histograms.xml

@@ -6009,6 +6009,14 @@ Therefore, the affected-histogram name has to have at least one dot in it.
   <summary>Complete time spent in DomDistiller's processing of a page.</summary>
 </histogram>
 
+<histogram name="DomDistiller.Time.DistillPage" units="milliseconds">
+  <owner>wychen@chromium.org</owner>
+  <summary>
+    Time spent in DistillPageImpl(), mostly ExecuteJavaScript(), including
+    compilation and execution.
+  </summary>
+</histogram>
+
 <histogram name="DomDistiller.Time.DocumentConstruction" units="milliseconds">
   <owner>yfriedman@chromium.org</owner>
   <summary>