blink: Find out how often a scrollable element has inline-end padding

As above. This adds two use-counters to give us web compat data for:
https://github.com/w3c/csswg-drafts/issues/129

The use counters will trigger if we have non-zero inline-end padding,
and we have block-level children (inline children already account for
padding), and are either "overflow: scroll" or "overflow: auto".

Change-Id: Idcee6d7aa6e5d970874352e82507fc5cce4ecc28
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2261404
Commit-Queue: Ian Kilpatrick <ikilpatrick@chromium.org>
Reviewed-by: David Grogan <dgrogan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781586}

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

@@ -2669,6 +2669,8 @@ enum WebFeature {
   kContentVisibilityAuto = 3332,
   kContentVisibilityHidden = 3333,
   kContentVisibilityHiddenMatchable = 3334,
+  kInlineOverflowAutoWithInlineEndPadding = 3335,
+  kInlineOverflowScrollWithInlineEndPadding = 3336,
 
   // 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/layout/layout_block.cc

@@ -503,6 +503,18 @@ void LayoutBlock::ComputeLayoutOverflow(LayoutUnit old_client_after_edge,
           LayoutUnit(1));
     AddLayoutOverflow(rect_to_apply);
     SetLayoutClientAfterEdge(old_client_after_edge);
+
+    if (PaddingEnd() && !ChildrenInline()) {
+      EOverflow overflow = StyleRef().OverflowInlineDirection();
+      if (overflow == EOverflow::kAuto) {
+        UseCounter::Count(GetDocument(),
+                          WebFeature::kInlineOverflowAutoWithInlineEndPadding);
+      } else if (overflow == EOverflow::kScroll) {
+        UseCounter::Count(
+            GetDocument(),
+            WebFeature::kInlineOverflowScrollWithInlineEndPadding);
+      }
+    }
   }
 }
 

tools/metrics/histograms/enums.xml

@@ -28063,6 +28063,8 @@ Called by update_use_counter_feature_enum.py.-->
   <int value="3332" label="ContentVisibilityAuto"/>
   <int value="3333" label="ContentVisibilityHidden"/>
   <int value="3334" label="ContentVisibilityHiddenMatchable"/>
+  <int value="3335" label="InlineOverflowAutoWithInlineEndPadding"/>
+  <int value="3336" label="InlineOverflowScrollWithInlineEndPadding"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">