Add histograms for script execution time for doc write 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 5ef617d9dfbd1d54d00646242fdddef5839a9407..5538cf4f7e62d3f61fe985f789b7ac7be6998c98 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -103253,7 +103253,6 @@ To add a new entry, add it with any value and run test to compute valid value.
       name="PageLoad.Clients.DocWrite.Block.Timing2.ParseDuration"/>

[gayane -on leave until 09-2017, 2016/09/16 16:59:43]

Can the suffix name="Background" on line 104030 go to the top before all the
affected histograms are defined ?

[Bryan McQuade, 2016/09/19 15:11:51]

ah, yes, not sure how things got re-ordered like this. moved it, thanks!

   <affected-histogram
       name="PageLoad.Clients.DocWrite.Evaluator.PaintTiming.NavigationToFirstContentfulPaint"/>
-  ;

[Charlie Harrison, 2016/09/15 15:28:59]

Should histograms.xml presubmit be robust to this?

   <affected-histogram
       name="PageLoad.Clients.DocWrite.Evaluator.ParseTiming.ParseBlockedOnScriptLoad"/>
   <affected-histogram
@@ -103399,6 +103398,10 @@ To add a new entry, add it with any value and run test to compute valid value.
   <affected-histogram
       name="PageLoad.PaintTiming.ParseStartToFirstContentfulPaint"/>
   <affected-histogram name="PageLoad.ParseTiming.NavigationToParseStart"/>
+  <affected-histogram
+      name="PageLoad.ParseTiming.ParseBlockedOnScriptExecution"/>
+  <affected-histogram
+      name="PageLoad.ParseTiming.ParseBlockedOnScriptExecutionFromDocumentWrite"/>

[gayane -on leave until 09-2017, 2016/09/16 16:59:43]

should these be PageLoad.Clients.DocWrite.Evaluator.ParseTiming.* instead of
PageLoad.ParseTiming.*

[Bryan McQuade, 2016/09/19 15:11:51]

ah, my mistake, i put the affected-histograms in the wrong block, thanks! i've
moved this to the 'clients' block. i also re-ordered this block to put the
<suffix> entries at the top.

   <affected-histogram name="PageLoad.ParseTiming.ParseBlockedOnScriptLoad"/>
   <affected-histogram
       name="PageLoad.ParseTiming.ParseBlockedOnScriptLoadFromDocumentWrite"/>