Apply BackgroundClip for pseudo custom scrollbar styles

According to the issue, background-clip was not applied to pseudo-styles.
Therefore, kBackgroundClip appended to target styles.

Bug: 1076508
Change-Id: I93595a14e3c4429faa56b7d060e3b8b9242b0518
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2586293Reviewed-by: Rune Lillesveen <futhark@chromium.org>
Reviewed-by: Mason Freed <masonfreed@chromium.org>
Commit-Queue: Seokho Song <0xdevssh@gmail.com>
Cr-Commit-Position: refs/heads/master@{#836975}

third_party/blink/renderer/core/html/forms/internal_popup_menu.cc

@@ -496,7 +496,7 @@ void InternalPopupMenu::AppendOwnerElementPseudoStyles(
       CSSPropertyID::kWidth,          CSSPropertyID::kBorderBottom,
       CSSPropertyID::kBorderLeft,     CSSPropertyID::kBorderRight,
       CSSPropertyID::kBorderTop,      CSSPropertyID::kBorderRadius,
-      CSSPropertyID::kBoxShadow};
+      CSSPropertyID::kBackgroundClip, CSSPropertyID::kBoxShadow};
 
   for (CSSPropertyID id : serialize_targets) {
     PagePopupClient::AddString(SerializeComputedStyleForProperty(style, id),

third_party/blink/web_tests/fast/forms/select-popup/popup-menu-appearance-custom-scrollbar.html

@@ -48,10 +48,12 @@
 
   select::-webkit-scrollbar-thumb {
     border-width: 3px;
-    border-color: black;
     border-style: solid;
-    border-radius: 10px;
-    background: gray;
+    border-color: transparent;
+    border-right-color: black;
+    border-top-color: black;
+    background-clip: padding-box;
+    background-color: gray;
     border-radius: 0 20px 0 10px;
     box-shadow: -1px 0 4px 1px inset white;
   }