Retire CSSOverscrollBehavior flag

overscroll-behavior shipped in M63
https://chromium-review.googlesource.com/797351
https://www.chromestatus.com/features/5734614437986304

BUG=789590

Change-Id: Ib5237ff2ccc2d2549aed6292520db0878658db45
Reviewed-on: https://chromium-review.googlesource.com/1165047Reviewed-by: Sandra Sun <sunyunjia@chromium.org>
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#581390}

third_party/blink/renderer/core/css/CSSProperties.json5

@@ -2420,7 +2420,6 @@
     {
       name: "overscroll-behavior-x",
       property_methods: ["CSSValueFromComputedStyleInternal"],
-      runtime_flag: "CSSOverscrollBehavior",
       field_template: "keyword",
       keywords: ["auto", "contain", "none"],
       default_value: "auto",
@@ -2430,7 +2429,6 @@
     {
       name: "overscroll-behavior-y",
       property_methods: ["CSSValueFromComputedStyleInternal"],
-      runtime_flag: "CSSOverscrollBehavior",
       field_template: "keyword",
       keywords: ["auto", "contain", "none"],
       default_value: "auto",
@@ -4868,7 +4866,6 @@
       name: "overscroll-behavior",
       longhands: ["overscroll-behavior-x", "overscroll-behavior-y"],
       property_methods: ["ParseShorthand", "CSSValueFromComputedStyleInternal"],
-      runtime_flag: "CSSOverscrollBehavior",
     },
     {
       name: "padding",

third_party/blink/renderer/core/dom/document.cc

@@ -2043,8 +2043,7 @@ void Document::PropagateStyleToViewport() {
   EOverscrollBehavior overscroll_behavior_y =
       overflow_style->OverscrollBehaviorY();
   using OverscrollBehaviorType = cc::OverscrollBehavior::OverscrollBehaviorType;
-  if (RuntimeEnabledFeatures::CSSOverscrollBehaviorEnabled() &&
-      IsInMainFrame()) {
+  if (IsInMainFrame()) {
     GetPage()->GetOverscrollController().SetOverscrollBehavior(
         cc::OverscrollBehavior(
             static_cast<OverscrollBehaviorType>(overscroll_behavior_x),

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -309,10 +309,6 @@
       name: "CSSOMSmoothScroll",
       status: "stable",
     },
-    {
-      name: "CSSOverscrollBehavior",
-      status: "stable",
-    },
     // Do not ship CSSPaintAPIArguments without shipping CSSVariables2 first.
     //
     // CSSPaintAPIArguments depends on parts of the the 'CSS Properties and