Add a histogram to roughly categorize MIME types for link rel=prefetch.

It isn't obvious what sort of resource this is typically used to
prefetch, and getting a rough estimate of this data from the field would
be helpful in understanding how this feature is used and how it can be
specified and evolved.

Some uncertainty arose in
w3c/resource-hints#86

Change-Id: I62d7e37c60aef7b5072d440a697642bfbe3816fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3546102
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/main@{#984502}
NOKEYCHECK=True
GitOrigin-RevId: 5d98fbfd97bb9d589364a22ce29b41f714bbc492

blink/renderer/core/loader/resource_load_observer_for_frame.cc

@@ -4,6 +4,7 @@
 
 #include "third_party/blink/renderer/core/loader/resource_load_observer_for_frame.h"
 
+#include "base/metrics/histogram_macros.h"
 #include "base/stl_util.h"
 #include "components/power_scheduler/power_mode_arbiter.h"
 #include "services/network/public/cpp/cors/cors_error_status.h"
@@ -12,6 +13,7 @@
 #include "third_party/blink/public/common/security/address_space_feature.h"
 #include "third_party/blink/public/mojom/frame/frame.mojom-blink.h"
 #include "third_party/blink/renderer/core/core_probes_inl.h"
+#include "third_party/blink/renderer/core/dom/events/event_target.h"
 #include "third_party/blink/renderer/core/frame/attribution_src_loader.h"
 #include "third_party/blink/renderer/core/frame/deprecation/deprecation.h"
 #include "third_party/blink/renderer/core/frame/frame_console.h"
@@ -173,6 +175,50 @@ void ResourceLoadObserverForFrame::DidChangePriority(
                                    identifier, priority);
 }
 
+namespace {
+
+// These values are persisted to logs. Entries should not be renumbered and
+// numeric values should never be reused.
+//
+// Must remain in sync with LinkPrefetchMimeType in
+// tools/metrics/histograms/enums.xml.
+enum class LinkPrefetchMimeType {
+  kUnknown = 0,
+  kHtml = 1,
+  kScript = 2,
+  kStyle = 3,
+  kFont = 4,
+  kImage = 5,
+  kMedia = 6,
+  kMaxValue = kMedia,
+};
+
+void LogLinkPrefetchMimeTypeHistogram(const AtomicString& mime) {
+  // Loosely based on https://mimesniff.spec.whatwg.org/#mime-type-groups.
+  // This could be done properly if needed, but this is just to gather
+  // approximate data.
+  LinkPrefetchMimeType type = LinkPrefetchMimeType::kUnknown;
+  if (mime == "text/html" || mime == "application/xhtml+xml") {
+    type = LinkPrefetchMimeType::kHtml;
+  } else if (mime == "application/javascript" || mime == "text/javascript") {
+    type = LinkPrefetchMimeType::kScript;
+  } else if (mime == "text/css") {
+    type = LinkPrefetchMimeType::kStyle;
+  } else if (mime.StartsWith("font/") || mime.StartsWith("application/font-") ||
+             mime == "application/vnd.ms-fontobject" ||
+             mime == "application/vnd.ms-opentype") {
+    type = LinkPrefetchMimeType::kFont;
+  } else if (mime.StartsWith("image/")) {
+    type = LinkPrefetchMimeType::kImage;
+  } else if (mime.StartsWith("audio/") || mime.StartsWith("video/") ||
+             mime == "application/ogg") {
+    type = LinkPrefetchMimeType::kMedia;
+  }
+  UMA_HISTOGRAM_ENUMERATION("Blink.Prefetch.LinkPrefetchMimeType", type);
+}
+
+}  // namespace
+
 void ResourceLoadObserverForFrame::DidReceiveResponse(
     uint64_t identifier,
     const ResourceRequest& request,
@@ -230,6 +276,9 @@ void ResourceLoadObserverForFrame::DidReceiveResponse(
     }
   }
 
+  if (resource->GetType() == ResourceType::kLinkPrefetch)
+    LogLinkPrefetchMimeTypeHistogram(response.MimeType());
+
   PreloadHelper::CanLoadResources resource_loading_policy =
       response_source == ResponseSource::kFromMemoryCache
           ? PreloadHelper::kDoNotLoadResources