Add breakdown histograms for injected script execution time

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 22aa63fd7b1a2163fab7c977e309f75639e3e6fb..3a20aec8a31bec4dd20db862350de4b85d443549 100644
--- a/tools/metrics/histograms/histograms.xml
+++ b/tools/metrics/histograms/histograms.xml
@@ -18591,6 +18591,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Extensions.InjectEnd_ExecutionTime" units="ms">
+  <owner>ksakamoto@chromium.org</owner>
+  <summary>
+    Time taken to execute all scripts at document end for one extension.
+  </summary>
+</histogram>
+
 <histogram name="Extensions.InjectEnd_ScriptCount">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>Number of scripts injected at document end by extensions.</summary>
@@ -18599,7 +18606,8 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 <histogram name="Extensions.InjectEnd_Time" units="ms">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
-    Time taken to inject all scripts at document end by extensions.
+    Time taken to inject all scripts at document end by extensions. Not reported
+    if scripts are executed asynchronously.
   </summary>
 </histogram>
 
@@ -18610,6 +18618,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   </summary>
 </histogram>
 
+<histogram name="Extensions.InjectIdle_ExecutionTime" units="ms">
+  <owner>ksakamoto@chromium.org</owner>
+  <summary>
+    Time taken to execute all scripts at document idle for one extension.
+  </summary>
+</histogram>
+
 <histogram name="Extensions.InjectIdle_ScriptCount">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>Number of scripts injected at document idle by extensions.</summary>
@@ -18618,7 +18633,8 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 <histogram name="Extensions.InjectIdle_Time" units="ms">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
-    Time taken to inject all scripts at document idle by extensions.
+    Time taken to inject all scripts at document idle by extensions. Not
+    reported if scripts are executed asynchronously.
   </summary>
 </histogram>
 
@@ -18642,6 +18658,13 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
   <summary>Number of css files injected by extensions.</summary>
 </histogram>
 
+<histogram name="Extensions.InjectStart_ExecutionTime" units="ms">
+  <owner>ksakamoto@chromium.org</owner>
+  <summary>
+    Time taken to execute all scripts at document start for one extension.
+  </summary>
+</histogram>
+
 <histogram name="Extensions.InjectStart_ScriptCount">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>Number of scripts injected at document start by extensions.</summary>
@@ -18650,7 +18673,8 @@ http://cs/file:chrome/histograms.xml - but prefer this file for new entries.
 <histogram name="Extensions.InjectStart_Time" units="ms">
   <owner>rdevlin.cronin@chromium.org</owner>
   <summary>
-    Time taken to inject css/scripts at document start by extensions.
+    Time taken to inject css/scripts at document start by extensions. Not
+    reported if scripts are executed asynchronously.
   </summary>
 </histogram>