RuntimeEnabledFeatures::PreferNonCompositedScrolling

Extracted from pdr@'s https://chromium-review.googlesource.com/c/chromium/src/+/2209993/3.
For testing non-composited-scrolling without depending on volatile
composited-scrolling trigger rules based on style.

Change-Id: I173e517da6539ca42336704c0a20942ef1c1e12b
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2213323
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Reviewed-by: Philip Rogers <pdr@chromium.org>
Reviewed-by: Sam Fortiner <samfort@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#772053}

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

@@ -2528,7 +2528,8 @@ bool PaintLayerScrollableArea::ComputeNeedsCompositedScrollingInternal(
   }
 
   if (!force_prefer_compositing_to_lcd_text &&
-      !LayerNodeMayNeedCompositedScrolling(layer_)) {
+      (RuntimeEnabledFeatures::PreferNonCompositedScrollingEnabled() ||
+       !LayerNodeMayNeedCompositedScrolling(layer_))) {
     return false;
   }
 

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1437,6 +1437,14 @@
     {
       name: "PreciseMemoryInfo",
     },
+    // Prefer not using composited scrolling. Composited scrolling will still
+    // be used if there are other reasons forcing compositing. For consistency,
+    // any code calling Settings::GetPreferCompositingToLCDTextEnabled() should
+    // ensure that this flag overrides the setting.
+    {
+      name: "PreferNonCompositedScrolling",
+      settable_from_internals: true,
+    },
     // This feature is deprecated and we are evangelizing affected sites.
     // See https://crbug.com/346236 for current status.
     {