Measure usage of `<embed>` and `<object>` for non-plugin content.

The conversation in https://github.com/whatwg/fetch/pull/948 makes me
curious about how locked-in we are to the existing behavior around both
`<embed>` and `<object>` when used for non-plugin content. They're
basically strange `<iframe>`s, and it might be reasonable to push them
closer towards _being_ `<iframe>` elements, or to push them further
away, depending on how usage numbers look.

Change-Id: I843ae37ae74033fdb3ea9b920ce3932004a067b0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1847291
Commit-Queue: Mike West <mkwst@chromium.org>
Reviewed-by: Daniel Vogelheim <vogelheim@chromium.org>
Cr-Commit-Position: refs/heads/master@{#703722}

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

@@ -2427,6 +2427,10 @@ enum WebFeature {
   kInvalidFragmentDirective = 3043,
   kContactsManagerGetProperties = 3044,
   kEvaluateScriptMovedBetweenElementDocuments = 3045,
+  kPluginElementLoadedDocument = 3046,
+  kPluginElementLoadedImage = 3047,
+  kPluginElementLoadedExternal = 3048,
+
 
   // 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/html/html_plugin_element.cc

@@ -53,6 +53,7 @@
 #include "third_party/blink/renderer/core/page/scrolling/scrolling_coordinator.h"
 #include "third_party/blink/renderer/platform/bindings/v8_per_isolate_data.h"
 #include "third_party/blink/renderer/platform/instrumentation/histogram.h"
+#include "third_party/blink/renderer/platform/instrumentation/use_counter.h"
 #include "third_party/blink/renderer/platform/loader/fetch/resource_request.h"
 #include "third_party/blink/renderer/platform/network/mime/mime_type_from_url.h"
 #include "third_party/blink/renderer/platform/network/mime/mime_type_registry.h"
@@ -581,6 +582,16 @@ bool HTMLPlugInElement::RequestObject(const PluginParameters& plugin_params) {
     ResetInstance();
   if (object_type == ObjectContentType::kFrame ||
       object_type == ObjectContentType::kImage || handled_externally) {
+    if (object_type == ObjectContentType::kFrame) {
+      UseCounter::Count(GetDocument(),
+                        WebFeature::kPluginElementLoadedDocument);
+    } else if (object_type == ObjectContentType::kImage) {
+      UseCounter::Count(GetDocument(), WebFeature::kPluginElementLoadedImage);
+    } else {
+      UseCounter::Count(GetDocument(),
+                        WebFeature::kPluginElementLoadedExternal);
+    }
+
     if (ContentFrame() && ContentFrame()->IsRemoteFrame()) {
       // During lazy reattaching, the plugin element loses EmbeddedContentView.
       // Since the ContentFrame() is not torn down the options here are to

tools/metrics/histograms/enums.xml

@@ -24867,6 +24867,9 @@ Called by update_net_error_codes.py.-->
   <int value="3043" label="InvalidFragmentDirective"/>
   <int value="3044" label="ContactsManagerGetProperties"/>
   <int value="3045" label="EvaluateScriptMovedBetweenElementDocuments"/>
+  <int value="3046" label="PluginElementLoadedDocument"/>
+  <int value="3047" label="PluginElementLoadedImage"/>
+  <int value="3048" label="PluginElementLoadedExternal"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">