Add a use counter for Body::body in a ServiceWorker

Add the use counter FetchBodyStreamInServiceWorker which is triggered
when the body getter is accessed in the Response class.

The purpose of the use counter is to evaluate the potential benefit of
Response body stream optimisations.

Change-Id: I625bb08712f6fc1996def8bdab52a449fc725f07
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1909675Reviewed-by: Yutaka Hirano <yhirano@chromium.org>
Commit-Queue: Adam Rice <ricea@chromium.org>
Cr-Commit-Position: refs/heads/master@{#714764}

third_party/blink/public/mojom/web_feature/web_feature.mojom

@@ -2467,6 +2467,8 @@ enum WebFeature {
   kByteLengthQueuingStrategyConstructor = 3083,
   kClassicDedicatedWorker = 3084,
   kModuleDedicatedWorker = 3085,
+  kFetchBodyStreamInServiceWorker = 3086,
+  kFetchBodyStreamOutsideServiceWorker = 3087,
 
   // Add new features immediately above this line. Don't change assigned
   // numbers of any item, and don't reuse removed slots.

third_party/blink/renderer/core/fetch/body.cc

@@ -15,6 +15,7 @@
 #include "third_party/blink/renderer/core/fetch/body_stream_buffer.h"
 #include "third_party/blink/renderer/core/fetch/fetch_data_loader.h"
 #include "third_party/blink/renderer/core/fileapi/blob.h"
+#include "third_party/blink/renderer/core/frame/web_feature.h"
 #include "third_party/blink/renderer/core/html/forms/form_data.h"
 #include "third_party/blink/renderer/core/typed_arrays/dom_array_buffer.h"
 #include "third_party/blink/renderer/core/typed_arrays/dom_typed_array.h"
@@ -349,6 +350,14 @@ ScriptPromise Body::text(ScriptState* script_state,
 }
 
 ReadableStream* Body::body() {
+  auto* execution_context = GetExecutionContext();
+  if (execution_context->IsServiceWorkerGlobalScope()) {
+    execution_context->CountUse(WebFeature::kFetchBodyStreamInServiceWorker);
+  } else {
+    execution_context->CountUse(
+        WebFeature::kFetchBodyStreamOutsideServiceWorker);
+  }
+
   if (!BodyBuffer())
     return nullptr;
   return BodyBuffer()->Stream();

tools/metrics/histograms/enums.xml

@@ -25436,6 +25436,8 @@ Called by update_net_error_codes.py.-->
   <int value="3083" label="ByteLengthQueuingStrategyConstructor"/>
   <int value="3084" label="ClassicDedicatedWorker"/>
   <int value="3085" label="ModuleDedicatedWorker"/>
+  <int value="3086" label="FetchBodyStreamInServiceWorker"/>
+  <int value="3087" label="FetchBodyStreamOutsideServiceWorker"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">