Add breakdown histograms for injected script execution time

extensions/renderer/script_injection.cc

Index: extensions/renderer/script_injection.cc
diff --git a/extensions/renderer/script_injection.cc b/extensions/renderer/script_injection.cc
index 519dce0ba520efa0ae83a0ab0ad8b745364942e7..b372a04edd9aab744470a4f2c9378ab9c7a7a3ec 100644
--- a/extensions/renderer/script_injection.cc
+++ b/extensions/renderer/script_injection.cc
@@ -323,8 +323,22 @@ void ScriptInjection::OnJsInjectionCompleted(
     base::TimeDelta elapsed) {
   DCHECK(!did_inject_js_);
 
-  if (injection_host_->id().type() == HostID::EXTENSIONS)
+  if (injection_host_->id().type() == HostID::EXTENSIONS) {
     UMA_HISTOGRAM_TIMES("Extensions.InjectedScriptExecutionTime", elapsed);
+    switch (run_location_) {
+      case UserScript::DOCUMENT_START:
+        UMA_HISTOGRAM_TIMES("Extensions.InjectStart_ExecutionTime", elapsed);

[Devlin, 2017/03/21 20:40:47]

I'd recommend using a histogram suffix here, and making these, e.g.,
Extensions.InjectionExecutionTime.DocumentStart, etc.

[Kunihiko Sakamoto, 2017/03/22 07:46:34]

Ah I've forgotten about histogram_suffixes.
Done, thanks!

+        break;
+      case UserScript::DOCUMENT_END:
+        UMA_HISTOGRAM_TIMES("Extensions.InjectEnd_ExecutionTime", elapsed);
+        break;
+      case UserScript::DOCUMENT_IDLE:
+        UMA_HISTOGRAM_TIMES("Extensions.InjectIdle_ExecutionTime", elapsed);
+        break;
+      default:
+        break;
+    }
+  }
 
   bool expects_results = injector_->ExpectsResults();
   if (expects_results) {