Implement Middle Click Autoscroll on all platforms not just Windows

third_party/WebKit/Source/web/WebRuntimeFeatures.cpp

Index: third_party/WebKit/Source/web/WebRuntimeFeatures.cpp
diff --git a/third_party/WebKit/Source/web/WebRuntimeFeatures.cpp b/third_party/WebKit/Source/web/WebRuntimeFeatures.cpp
index 1a9e7e8e690f9b337dbd010ea528840b436b9ee3..1a690eaa234d055172b07dfef33fdda6f7ba2525 100644
--- a/third_party/WebKit/Source/web/WebRuntimeFeatures.cpp
+++ b/third_party/WebKit/Source/web/WebRuntimeFeatures.cpp
@@ -215,6 +215,11 @@ void WebRuntimeFeatures::enablePagePopup(bool enable)
     RuntimeEnabledFeatures::setPagePopupEnabled(enable);
 }
 
+void WebRuntimeFeatures::enableMiddleClickAutoscroll(bool enable)
+{
+    RuntimeEnabledFeatures::setMiddleClickAutoscrollEnabled(enable);
+}
+
 void WebRuntimeFeatures::enableParseHTMLOnMainThread(bool enable)
 {
     RuntimeEnabledFeatures::setParseHTMLOnMainThreadEnabled(enable);