Plumb pinch virtual viewport flag to CC.

cc/base/switches.cc

Index: cc/base/switches.cc
diff --git a/cc/base/switches.cc b/cc/base/switches.cc
index 9886ae80f8b9de4fbb1dc4344a40f620378e61c2..57984af68482eb7bd6b1e6077bc88fdb448e54cf 100644
--- a/cc/base/switches.cc
+++ b/cc/base/switches.cc
@@ -81,6 +81,9 @@ const char kCompositeToMailbox[] = "composite-to-mailbox";
 const char kStrictLayerPropertyChangeChecking[] =
     "strict-layer-property-change-checking";
 
+// Virtual viewport for fixed-position elements, scrollbars during pinch.
+const char kEnablePinchVirtualViewport[] = "enable-pinch-virtual-viewport";
+
 const char kEnablePartialSwap[] = "enable-partial-swap";
 // Disable partial swap which is needed for some OpenGL drivers / emulators.
 const char kUIDisablePartialSwap[] = "ui-disable-partial-swap";