Plumb experiment flags through page_load_metrics and add a new observer

tools/metrics/histograms/histograms.xml

Index: tools/metrics/histograms/histograms.xml
diff --git a/tools/metrics/histograms/histograms.xml b/tools/metrics/histograms/histograms.xml
index cdd10ea8eb07cf55622420a694e0131d6e2f92e8..518bcd18b3202aa0f579a6618f31e5d84a3071ca 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -34566,6 +34566,15 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="PageLoad.Timing2.ParseStartToFirstContentfulPaint" units="ms">
+  <owner>bmcquade@chromium.org</owner>
+  <owner>csharrison@chromium.org</owner>
+  <summary>
+    Measures the time from when the HTML parser started, to when the page first
+    paints content.
+  </summary>
+</histogram>
+
 <histogram name="PartitionAlloc.CommittedSize" units="MB">
   <owner>haraken@chromium.org</owner>
   <summary>
@@ -90261,6 +90270,9 @@ To add a new entry, add it with any value and run test to compute valid value.
   <suffix name="Clients.FromGWS"
       label="PageLoadMetrics that are a result of a navigation from a Google
              web search"/>
+  <suffix name="Clients.DocWrite.Evaluator"
+      label="PageLoadMetrics from a page that could have been evaluated by
+             the DocumentWriteEvaluator"/>
   <affected-histogram
       name="PageLoad.Timing2.DOMLoadingToDOMContentLoadedEventFired"/>
   <affected-histogram name="PageLoad.Timing2.DOMLoadingToFirstContentfulPaint"/>
@@ -90272,6 +90284,11 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram name="PageLoad.Timing2.NavigationToFirstPaint"/>
   <affected-histogram name="PageLoad.Timing2.NavigationToFirstTextPaint"/>
   <affected-histogram name="PageLoad.Timing2.NavigationToLoadEventFired"/>
+  <affected-histogram name="PageLoad.Timing2.ParseBlockedOnScriptLoad"/>
+  <affected-histogram
+      name="PageLoad.Timing2.ParseBlockedOnScriptLoad.ParseComplete"/>
+  <affected-histogram name="PageLoad.Timing2.ParseDuration"/>
+  <affected-histogram name="PageLoad.Timing2.ParseStartToFirstContentfulPaint"/>
 </histogram_suffixes>
 
 <histogram_suffixes name="PageLoadType">