Remove SquashAfterPaint experiment remnants

Change-Id: I379af4d45d9815de894670e4d602263a48f21d6f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2284054
Commit-Queue: Philip Rogers <pdr@chromium.org>
Commit-Queue: Stefan Zager <szager@chromium.org>
Auto-Submit: Philip Rogers <pdr@chromium.org>
Reviewed-by: Stefan Zager <szager@chromium.org>
Cr-Commit-Position: refs/heads/master@{#785950}

third_party/blink/renderer/platform/graphics/paint/graphics_layer_display_item.h

@@ -18,10 +18,6 @@ class GraphicsLayer;
 // Before CompositeAfterPaint, this is a placeholder for a GraphicsLayer
 // allocated before paint. With CompositeAfterPaint, this class will be
 // obsolete and should be removed.
-//
-// Before SquashAfterPaint, a cc::Layer will always be allocated to draw the
-// GraphicsLayer's contents. With SquashAfter Paint, that may not be true; the
-// GraphicsLayer may end up getting squashed by PaintArtifactCompositor.
 class PLATFORM_EXPORT GraphicsLayerDisplayItem final : public DisplayItem {
  public:
   GraphicsLayerDisplayItem(const GraphicsLayer&);

third_party/blink/renderer/platform/runtime_enabled_features.json5

@@ -1713,9 +1713,6 @@
       name: "SmsReceiver",
       status: {"default": "experimental", "Android": "stable"},
     },
-    {
-      name: "SquashAfterPaint",
-    },
     // Used as argument in attribute of stable-release functions/interfaces
     // where a runtime-enabled feature name is required for correct IDL syntax.
     // This is a global flag; do not change its status.