Add Async Script Count UKM

This CL adds UKM to count the number of async scripts associated with
a given mainframe document.

Google-internal UKM privacy review:
https://docs.google.com/document/d/1bKyDRF5BAQpPJ-bRyKs2dgqBTJI43b63kOULO-HIuXM/edit

Bug: 1086227
Change-Id: Iaaee8c756952613c2ea58416b453a98a0843773f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2209118
Commit-Queue: Dominic Farolino <dom@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#771611}

third_party/blink/renderer/core/dom/document.cc

@@ -42,6 +42,7 @@
 #include "mojo/public/cpp/bindings/pending_remote.h"
 #include "mojo/public/cpp/bindings/remote.h"
 #include "mojo/public/mojom/base/text_direction.mojom-blink.h"
+#include "services/metrics/public/cpp/metrics_utils.h"
 #include "services/metrics/public/cpp/mojo_ukm_recorder.h"
 #include "services/metrics/public/cpp/ukm_builders.h"
 #include "services/metrics/public/cpp/ukm_source_id.h"
@@ -6855,9 +6856,24 @@ void SetOriginTrialFreezePolicy(
 
 }  // namespace
 
+void Document::RecordAsyncScriptCount() {
+  // Only record async script count for mainframe documents, as per UKM policy.
+  if (!IsInMainFrame())
+    return;
+
+  auto* recorder = UkmRecorder();
+  DCHECK(recorder);
+  DCHECK(UkmSourceID() != ukm::kInvalidSourceId);
+  ukm::builders::Blink_Script_AsyncScripts(UkmSourceID())
+      .SetAsyncScriptCount(
+          ukm::GetExponentialBucketMin(async_script_count_, 1.3))
+      .Record(recorder);
+}
+
 void Document::FinishedParsing() {
   DCHECK(!GetScriptableDocumentParser() || !parser_->IsParsing());
   DCHECK(!GetScriptableDocumentParser() || ready_state_ != kLoading);
+  RecordAsyncScriptCount();
   SetParsingState(kInDOMContentLoaded);
   DocumentParserTiming::From(*this).MarkParserStop();
 

third_party/blink/renderer/core/dom/document.h

@@ -1686,6 +1686,9 @@ class CORE_EXPORT Document : public ContainerNode,
   FontPreloadManager& GetFontPreloadManager() { return font_preload_manager_; }
   void FontPreloadingFinishedOrTimedOut();
 
+  void IncrementAsyncScriptCount() { async_script_count_++; }
+  void RecordAsyncScriptCount();
+
   void SetFindInPageActiveMatchNode(Node*);
   const Node* GetFindInPageActiveMatchNode() const;
 
@@ -2269,6 +2272,8 @@ class CORE_EXPORT Document : public ContainerNode,
 
   FontPreloadManager font_preload_manager_;
 
+  int async_script_count_ = 0;
+
   WeakMember<Node> find_in_page_active_match_node_;
 };
 

third_party/blink/renderer/core/script/script_loader.cc

@@ -842,6 +842,8 @@ bool ScriptLoader::PrepareScript(const TextPosition& script_start_position,
     // script is ready, execute the script block and then remove the element
     // from the set of scripts that will execute as soon as possible.</spec>
     pending_script_ = TakePendingScript(ScriptSchedulingType::kAsync);
+    // This is for the UKM count of async scripts in a document.
+    context_window->document()->IncrementAsyncScriptCount();
     // TODO(hiroshige): Here the context document is used as "node document"
     // while Step 14 uses |elementDocument| as "node document". Fix this.
     context_window->document()->GetScriptRunner()->QueueScriptForExecution(

tools/metrics/ukm/ukm.xml

@@ -1835,6 +1835,21 @@ be describing additional metrics about the same event.
   </metric>
 </event>
 
+<event name="Blink.Script.AsyncScripts" singular="True">
+  <owner>dom@chromium.org</owner>
+  <owner>chrome-loading@google.com</owner>
+  <summary>
+    Metrics associated with the async script count associated with a document,
+    recorded when the HTML parsing of a mainframe has completed.
+  </summary>
+  <metric name="AsyncScriptCount">
+    <summary>
+      The total number of async scripts associated with a document at the time
+      parsing has finished.
+    </summary>
+  </metric>
+</event>
+
 <event name="Blink.UpdateTime">
   <owner>schenney@chromium.org</owner>
   <owner>paint-dev@chromium.org</owner>