Add a WebLayer:setScrollCompensationAdjustment between Blink and CC

public/platform/WebLayer.h

Index: public/platform/WebLayer.h
diff --git a/public/platform/WebLayer.h b/public/platform/WebLayer.h
index 74bf4978e4e0f83a0095f5f457519a45f76952b6..f4fcc94d70a8173e5b168e517cfb5ef045cb8fc7 100644
--- a/public/platform/WebLayer.h
+++ b/public/platform/WebLayer.h
@@ -164,6 +164,18 @@ public:
     // Scrolling
     virtual void setScrollPositionDouble(WebDoublePoint) = 0;
     virtual WebDoublePoint scrollPositionDouble() const = 0;
+    // Blink tells cc the scroll offset through setScrollPositionDouble() using
+    // floating precision but it currently can only position cc layers at integer
+    // boundary. So Blink needs to also call setScrollCompensationAdjustment()
+    // to tell cc what's the part of the scroll offset that Blink doesn't handle
+    // but cc needs to take into consideration, e.g. compensating
+    // for fixed-position layer that's positioned in Blink using only integer scroll
+    // offset.
+    // We make this call explicit, instead of letting cc to infer the fractional part
+    // from the scroll offset, to be clear that this is Blink's limitation. Once
+    // Blink can fully handle fractional scroll offset, it can stop calling
+    // this function and cc side would just work.
+    virtual void setScrollCompensationAdjustment(WebDoublePoint) = 0;
 
     // To set a WebLayer as scrollable we must specify the corresponding clip layer.
     virtual void setScrollClipLayer(WebLayer*) = 0;