Add Blink-side flag for enabling/disabling layer squashing.

Source/core/testing/InternalSettings.h

Index: Source/core/testing/InternalSettings.h
diff --git a/Source/core/testing/InternalSettings.h b/Source/core/testing/InternalSettings.h
index ec312063d38e1786c15004eb0352da4dae130cb9..cc064baa413be3ef6432f977862750ff77d39ac1 100644
--- a/Source/core/testing/InternalSettings.h
+++ b/Source/core/testing/InternalSettings.h
@@ -67,6 +67,7 @@ public:
         bool m_shouldDisplayTextDescriptions;
         String m_defaultVideoPosterURL;
         bool m_originalCompositorDrivenAcceleratedScrollEnabled;
+        bool m_originalLayerSquashingEnabled;
         bool m_originalPasswordGenerationDecorationEnabled;
     };
 
@@ -104,6 +105,10 @@ public:
     // overflow scroll is ready (crbug.com/254111).
     void setCompositorDrivenAcceleratedScrollingEnabled(bool, ExceptionState&);
 
+    // FIXME: This is a temporary flag and should be removed once squashing is
+    // ready (crbug.com/261605).
+    void setLayerSquashingEnabled(bool, ExceptionState&);
+
     // FIXME: The following are RuntimeEnabledFeatures and likely
     // cannot be changed after process start. These setters should
     // be removed or moved onto internals.runtimeFlags: