[ES6 modules] Add UseCounter for preparing a module script

Bug: 594639
Change-Id: Iabb2f3780d21c0f896073e6cad6178deced61e83
Reviewed-on: https://chromium-review.googlesource.com/575797Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Commit-Queue: Kouhei Ueno <kouhei@chromium.org>
Cr-Commit-Position: refs/heads/master@{#487432}

third_party/WebKit/Source/core/dom/ScriptLoader.cpp

@@ -45,6 +45,7 @@
 #include "core/events/Event.h"
 #include "core/frame/LocalFrame.h"
 #include "core/frame/SubresourceIntegrity.h"
+#include "core/frame/UseCounter.h"
 #include "core/frame/csp/ContentSecurityPolicy.h"
 #include "core/html/imports/HTMLImport.h"
 #include "core/html/parser/HTMLParserIdioms.h"
@@ -357,6 +358,9 @@ bool ScriptLoader::PrepareScript(const TextPosition& script_start_position,
   ParserDisposition parser_state =
       IsParserInserted() ? kParserInserted : kNotParserInserted;
 
+  if (GetScriptType() == ScriptType::kModule)
+    UseCounter::Count(*context_document, WebFeature::kPrepareModuleScript);
+
   // 21. "If the element has a src content attribute, run these substeps:"
   if (element_->HasSourceAttribute()) {
     // 21.1. Let src be the value of the element's src attribute.

third_party/WebKit/public/platform/web_feature.mojom

@@ -1597,6 +1597,7 @@ enum WebFeature {
   kRequestMIDIAccessDisallowedByFeaturePolicyInCrossOriginIframe = 2059,
   kMediaSourceKeyframeTimeGreaterThanDependant = 2060,
   kMediaSourceMuxedSequenceMode = 2061,
+  kPrepareModuleScript = 2062,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

tools/metrics/histograms/enums.xml

@@ -15898,6 +15898,7 @@ uploading your change for review.  These are checked by presubmit scripts.
       label="RequestMIDIAccessDisallowedByFeaturePolicyInCrossOriginIframe"/>
   <int value="2060" label="MediaSourceKeyframeTimeGreaterThanDependant"/>
   <int value="2061" label="MediaSourceMuxedSequenceMode"/>
+  <int value="2062" label="PrepareModuleScript"/>
 </enum>
 
 <enum name="FeedbackSource">