Add a use counter for navigations targeted at _current

It's not in the spec, FF doesn't support it. Hopefully we can remove it.

Change-Id: Iaa9b78245d3513a2574570359f02922da09b5ed6
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1536010
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Auto-Submit: Nate Chapin <japhet@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644132}

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

@@ -2245,6 +2245,7 @@ enum WebFeature {
   kRefreshHeader = 2832,
   kSearchEventFired = 2833,
   kIdleDetectionStart = 2834,
+  kTargetCurrent = 2835,
 
   // 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/page/frame_tree.cc

@@ -181,6 +181,12 @@ Frame* FrameTree::Find(const AtomicString& name) const {
   // Named frame lookup should always be relative to a local frame.
   DCHECK(IsA<LocalFrame>(this_frame_.Get()));
 
+  if (EqualIgnoringASCIICase(name, "_current")) {
+    UseCounter::Count(
+        blink::DynamicTo<blink::LocalFrame>(this_frame_.Get())->GetDocument(),
+        WebFeature::kTargetCurrent);
+  }
+
   if (EqualIgnoringASCIICase(name, "_self") ||
       EqualIgnoringASCIICase(name, "_current") || name.IsEmpty())
     return this_frame_;

tools/metrics/histograms/enums.xml

@@ -22611,6 +22611,7 @@ Called by update_net_error_codes.py.-->
   <int value="2832" label="RefreshHeader"/>
   <int value="2833" label="SearchEventFired"/>
   <int value="2834" label="IdleDetectionStart"/>
+  <int value="2835" label="TargetCurrent"/>
 </enum>
 
 <enum name="FeaturePolicyFeature">