Add UMA for script scheduling type

Bug: 856922
Change-Id: Ia294d6c2a1b418b9a7f46ab41c574a4eace8c626
Reviewed-on: https://chromium-review.googlesource.com/1116508
Commit-Queue: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Reviewed-by: Ilya Sherman <isherman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573816}

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

@@ -47,6 +47,7 @@
 #include "third_party/blink/renderer/core/script/script_element_base.h"
 #include "third_party/blink/renderer/core/script/script_runner.h"
 #include "third_party/blink/renderer/core/svg_names.h"
+#include "third_party/blink/renderer/platform/histogram.h"
 #include "third_party/blink/renderer/platform/loader/fetch/access_control_status.h"
 #include "third_party/blink/renderer/platform/loader/fetch/fetch_parameters.h"
 #include "third_party/blink/renderer/platform/loader/fetch/resource_fetcher.h"
@@ -730,6 +731,11 @@ PendingScript* ScriptLoader::TakePendingScript(
     ScriptSchedulingType scheduling_type) {
   CHECK(prepared_pending_script_);
 
+  DEFINE_STATIC_LOCAL(
+      EnumerationHistogram, scheduling_type_histogram,
+      ("Blink.Script.SchedulingType", kLastScriptSchedulingType + 1));
+  scheduling_type_histogram.Count(static_cast<int>(scheduling_type));
+
   switch (scheduling_type) {
     case ScriptSchedulingType::kAsync:
     case ScriptSchedulingType::kInOrder:

third_party/blink/renderer/core/script/script_scheduling_type.h

@@ -12,6 +12,9 @@ namespace blink {
 // In the spec, this is determined which clause of Step 25 of
 // https://html.spec.whatwg.org/multipage/scripting.html#prepare-a-script
 // is taken.
+//
+// The enum values are used in histograms and thus do not change existing
+// enum values when modifying.
 enum class ScriptSchedulingType {
   // Because the sheduling type is determined slightly after PendingScript
   // creation, it is set to kNotSet before ScriptLoader::TakePendingScript()
@@ -62,6 +65,9 @@ enum class ScriptSchedulingType {
   kImmediate
 };
 
+static const int kLastScriptSchedulingType =
+    static_cast<int>(ScriptSchedulingType::kImmediate);
+
 }  // namespace blink
 
 #endif

tools/metrics/histograms/enums.xml

@@ -41861,6 +41861,16 @@ Called by update_net_trust_anchors.py.-->
   <int value="9" label="Rollback: SBER2 absent so SBER1 must be cleared"/>
 </enum>
 
+<enum name="ScriptSchedulingType">
+  <int value="0" label="NotSet"/>
+  <int value="1" label="Defer"/>
+  <int value="2" label="ParserBlocking"/>
+  <int value="3" label="ParserBlockingInline"/>
+  <int value="4" label="InOrder"/>
+  <int value="5" label="Async"/>
+  <int value="6" label="Immediate"/>
+</enum>
+
 <enum name="ScrollAnchorRestorationStatus">
   <int value="0" label="SUCCESS"/>
   <int value="1" label="FAILED_NO_MATCHES"/>

tools/metrics/histograms/histograms.xml

@@ -8493,6 +8493,16 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="Blink.Script.SchedulingType" enum="ScriptSchedulingType"
+    expires_after="2018-12-31">
+  <owner>kouhei@chromium.org</owner>
+  <owner>hiroshige@chromium.org</owner>
+  <summary>
+    Number of script elements for each scheduling type, recorded for each
+    successful #prepare-a-script.
+  </summary>
+</histogram>
+
 <histogram name="Blink.ScriptValueSerializer.DOMWrapperCount" units="objects">
   <obsolete>
     Used to justify distribution of serialized data in the wild, but unlikely to