Implement Middle Click Autoscroll on all platforms not just Windows

third_party/WebKit/Source/core/input/KeyboardEventManager.cpp

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "KeyboardEventManager.h"
 
 #include "core/dom/Element.h"
 #include "core/editing/Editor.h"
 #include "core/events/KeyboardEvent.h"
 #include "core/html/HTMLDialogElement.h"
@@ skipping 67 matching lines @@
 }
 
 WebInputEventResult KeyboardEventManager::keyEvent(
     const WebKeyboardEvent& initialKeyEvent)
 {
     m_frame->chromeClient().clearToolTip();
 
     if (initialKeyEvent.windowsKeyCode == VK_CAPITAL)
         capsLockStateMayHaveChanged();
 
-#if OS(WIN)
-    if (m_scrollManager->panScrollInProgress()) {
-        // If a key is pressed while the panScroll is in progress then we want to stop
+    if (m_scrollManager->middleClickAutoscrollInProgress()) {
+        DCHECK(RuntimeEnabledFeatures::middleClickAutoscrollEnabled());
+        // If a key is pressed while the middleClickAutoscroll is in progress then we want to stop
         if (initialKeyEvent.type == WebInputEvent::KeyDown || initialKeyEvent.type == WebInputEvent::RawKeyDown)
             m_scrollManager->stopAutoscroll();
 
         // If we were in panscroll mode, we swallow the key event
         return WebInputEventResult::HandledSuppressed;
     }
-#endif
 
     // Check for cases where we are too early for events -- possible unmatched key up
     // from pressing return in the location bar.
     Node* node = eventTargetNodeForDocument(m_frame->document());
     if (!node)
         return WebInputEventResult::NotHandled;
 
     UserGestureIndicator gestureIndicator(DefinitelyProcessingUserGesture);
 
     // In IE, access keys are special, they are handled after default keydown processing, but cannot be canceled - this is hard to match.
@@ skipping 248 matching lines @@
     if (currentModifiers & ::cmdKey)
         modifiers |= WebInputEvent::MetaKey;
 #else
     // TODO(crbug.com/538289): Implement on other platforms.
     return static_cast<WebInputEvent::Modifiers>(0);
 #endif
     return static_cast<WebInputEvent::Modifiers>(modifiers);
 }
 
 } // namespace blink