Add UMA to see impact of changing overscroll behavior root propagation

This CL follows the pattern of:
https://chromium-review.googlesource.com/c/chromium/src/+/1715906

That is, in order to change the viewport style propagation logic
for overscroll behavior, we want to add use counters to understand
the potential breakage.

Bug: 954423
Change-Id: I410aa665873167ffe523911a823e9b47cd117018
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2560396
Commit-Queue: Xida Chen <xidachen@chromium.org>
Reviewed-by: David Bokan <bokan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#831204}

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

@@ -3062,6 +3062,7 @@ enum WebFeature {
   kAddSourceBufferUsingConfig = 3733,
   kChangeTypeUsingConfig = 3734,
   kV8SourceBuffer_AppendEncodedChunks_Method = 3735,
+  kOversrollBehaviorOnViewportBreaks = 3736,
 
   // 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/dom/document.cc

@@ -2273,6 +2273,22 @@ void Document::PropagateStyleToViewport() {
     PROPAGATE_FROM(overflow_style, OverscrollBehaviorY, SetOverscrollBehaviorY,
                    EOverscrollBehavior::kAuto);
 
+    // Counts any time overscroll behavior break if we change its viewport
+    // propagation logic. Overscroll behavior only breaks if body has non-none
+    // type that is different from the document one.
+    // TODO(954423): Remove once propagation logic change is complete.
+    if (document_element_style && body_style) {
+      bool overscroll_behavior_is_different =
+          body_style->OverscrollBehaviorX() !=
+              document_element_style->OverscrollBehaviorX() ||
+          body_style->OverscrollBehaviorY() !=
+              document_element_style->OverscrollBehaviorY();
+      if (overscroll_behavior_is_different) {
+        UseCounter::Count(*this,
+                          WebFeature::kOversrollBehaviorOnViewportBreaks);
+      }
+    }
+
     EOverflow overflow_x = EOverflow::kAuto;
     EOverflow overflow_y = EOverflow::kAuto;
     EOverflowAnchor overflow_anchor = EOverflowAnchor::kAuto;

tools/metrics/histograms/enums.xml

@@ -30356,6 +30356,7 @@ Called by update_use_counter_feature_enum.py.-->
   <int value="3733" label="AddSourceBufferUsingConfig"/>
   <int value="3734" label="ChangeTypeUsingConfig"/>
   <int value="3735" label="V8SourceBuffer_AppendEncodedChunks_Method"/>
+  <int value="3736" label="OversrollBehaviorOnViewportBreaks"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">