Support buttons attribute for drag event (blink side).

Source/web/WebViewImpl.cpp

Index: Source/web/WebViewImpl.cpp
diff --git a/Source/web/WebViewImpl.cpp b/Source/web/WebViewImpl.cpp
index 06e1c3efd3a9d6a137123b869811287b1502407b..4cea0dc13af1c44ce4b6fa863bb28d9241d45ae3 100644
--- a/Source/web/WebViewImpl.cpp
+++ b/Source/web/WebViewImpl.cpp
@@ -244,6 +244,12 @@ static int webInputEventKeyStateToPlatformEventKeyState(int webInputEventKeyStat
         platformEventKeyState = platformEventKeyState | PlatformEvent::AltKey;
     if (webInputEventKeyState & WebInputEvent::MetaKey)
         platformEventKeyState = platformEventKeyState | PlatformEvent::MetaKey;
+    if (webInputEventKeyState & WebInputEvent::LeftButtonDown)
+        platformEventKeyState = platformEventKeyState | PlatformEvent::LeftButtonDown;
+    if (webInputEventKeyState & WebInputEvent::MiddleButtonDown)
+        platformEventKeyState = platformEventKeyState | PlatformEvent::MiddleButtonDown;
+    if (webInputEventKeyState & WebInputEvent::RightButtonDown)
+        platformEventKeyState = platformEventKeyState | PlatformEvent::RightButtonDown;
     return platformEventKeyState;
 }