Selectively disable rubber banding on mac.

public/web/WebInputEvent.h

Index: public/web/WebInputEvent.h
diff --git a/public/web/WebInputEvent.h b/public/web/WebInputEvent.h
index 819c62e1928b36c0c834fab1f5666d723e346cdd..cfa6b34756eddf2cede56b600f4b73dc6823da59 100644
--- a/public/web/WebInputEvent.h
+++ b/public/web/WebInputEvent.h
@@ -361,6 +361,24 @@ public:
     Phase phase;
     Phase momentumPhase;
 
+    // See comment at the top of the file for why an int is used here.
+    // Rubberbanding is an OSX visual effect. When a user scrolls the content
+    // area with a track pad, and the content area is already at its limit in
+    // the direction being scrolled, the entire content area is allowed to
+    // scroll slightly off screen, revealing a grey background. When the user
+    // lets go, the content area snaps back into place. Blink is responsible
+    // for this rubberbanding effect, but the embedder may wish to disable
+    // rubber banding in the left or right direction, if the scroll should have
+    // an alternate effect. The common case is that a scroll in the left or
+    // right directions causes a back or forwards navigation, respectively.
+    //
+    // These flags prevent rubber banding from starting in a given direction,
+    // but have no effect on an ongoing rubber banding. A rubber banding that
+    // started in the vertical direction is allowed to continue in the right
+    // direction, even if canRubberbandRight is 0.
+    int canRubberbandLeft;
+    int canRubberbandRight;
+
     WebMouseWheelEvent(unsigned sizeParam = sizeof(WebMouseWheelEvent))
         : WebMouseEvent(sizeParam)
         , deltaX(0.0f)
@@ -373,6 +391,8 @@ public:
         , hasPreciseScrollingDeltas(false)
         , phase(PhaseNone)
         , momentumPhase(PhaseNone)
+        , canRubberbandLeft(true)
+        , canRubberbandRight(true)
     {
     }
 };