Add UMA for custom scrollbar buttons in reversed direction

We support custom scrollbar buttons in reversed direction (e.g. the
backward scroll button at the bottom of a vertical scrollbar with
-webkit-scrollbar-button:decrement:end). Count it to see if we can
remove it to reduce complexity of custom scrollbars.

Bug: 1009063
Change-Id: If54d8c0deaae44cc9e40a21c55dfbe0e6c3b48a4
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1830217
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#700907}

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

@@ -2408,15 +2408,19 @@ enum WebFeature {
   kDeprecatedFileSystemRead = 3025,
   kDeprecatedFileSystemWrite = 3026,
   kPointerLockUnadjustedMovement = 3027,
+  // The above items are available in M78 branch.
+
   kCreateObjectBlob = 3028,
-  kQuotaRead = 3029,
+  kQuotaRead = 3029,  // available in M78 branch.
   kDelegateFocus = 3030,
   kDelegateFocusNotFirstInFlatTree = 3031,
   kThirdPartySharedWorker = 3032,
   kThirdPartyBroadcastChannel = 3033,
   kMediaSourceGroupEndTimestampDecreaseWithinMediaSegment = 3034,
-  kTextFragmentAnchorTapToDismiss = 3035,
+  kTextFragmentAnchorTapToDismiss = 3035,  // available in M78 branch.
   kXRIsSessionSupported = 3036,
+  kScrollbarUseScrollbarButtonReversedDirection = 3037,
+  kCSSSelectorPseudoScrollbarButtonReversedDirection = 3038,
 
   // 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_scrollbar_part.cc

@@ -47,9 +47,13 @@ LayoutScrollbarPart::LayoutScrollbarPart(ScrollableArea* scrollable_area,
 
 static void RecordScrollbarPartStats(Document& document, ScrollbarPart part) {
   switch (part) {
-    case kBackButtonStartPart:
-    case kForwardButtonStartPart:
     case kBackButtonEndPart:
+    case kForwardButtonStartPart:
+      UseCounter::Count(
+          document,
+          WebFeature::kCSSSelectorPseudoScrollbarButtonReversedDirection);
+      U_FALLTHROUGH;
+    case kBackButtonStartPart:
     case kForwardButtonEndPart:
       UseCounter::Count(document,
                         WebFeature::kCSSSelectorPseudoScrollbarButton);

third_party/blink/renderer/core/paint/paint_layer_scrollable_area.cc

@@ -2839,9 +2839,13 @@ void PaintLayerScrollableArea::DidScrollWithScrollbar(
     WebInputEvent::Type type) {
   WebFeature scrollbar_use_uma;
   switch (part) {
-    case kBackButtonStartPart:
-    case kForwardButtonStartPart:
     case kBackButtonEndPart:
+    case kForwardButtonStartPart:
+      UseCounter::Count(
+          GetLayoutBox()->GetDocument(),
+          WebFeature::kScrollbarUseScrollbarButtonReversedDirection);
+      U_FALLTHROUGH;
+    case kBackButtonStartPart:
     case kForwardButtonEndPart:
       scrollbar_use_uma =
           (orientation == kVerticalScrollbar

tools/metrics/histograms/enums.xml

@@ -24752,6 +24752,8 @@ Called by update_net_error_codes.py.-->
       label="MediaSourceGroupEndTimestampDecreaseWithinMediaSegment"/>
   <int value="3035" label="TextFragmentAnchorTapToDismiss"/>
   <int value="3036" label="XRIsSessionSupported"/>
+  <int value="3037" label="ScrollbarUseScrollbarButtonReversedDirection"/>
+  <int value="3038" label="CSSSelectorPseudoScrollbarButtonReversedDirection"/>
 </enum>
 
 <enum name="FeaturePolicyAllowlistType">