Implement MouseEvent buttons attribute.

Source/web/WebInputEventConversion.cpp

Index: Source/web/WebInputEventConversion.cpp
diff --git a/Source/web/WebInputEventConversion.cpp b/Source/web/WebInputEventConversion.cpp
index 09586f45a73f41cbb1bd634171343c8e73832508..2e25342b14e2d0f9121adf97091b4a45dd17bae5 100644
--- a/Source/web/WebInputEventConversion.cpp
+++ b/Source/web/WebInputEventConversion.cpp
@@ -104,11 +104,23 @@ PlatformMouseEventBuilder::PlatformMouseEventBuilder(Widget* widget, const WebMo
         m_modifiers |= PlatformEvent::AltKey;
     if (e.modifiers & WebInputEvent::MetaKey)
         m_modifiers |= PlatformEvent::MetaKey;
+    if (e.modifiers & WebInputEvent::LeftButtonDown)
+        m_modifiers |= PlatformEvent::LeftButtonDown;
+    if (e.modifiers & WebInputEvent::MiddleButtonDown)
+        m_modifiers |= PlatformEvent::MiddleButtonDown;
+    if (e.modifiers & WebInputEvent::RightButtonDown)
+        m_modifiers |= PlatformEvent::RightButtonDown;
 
     m_modifierFlags = e.modifiers;
     m_timestamp = e.timeStampSeconds;
     m_clickCount = e.clickCount;
 
+    const unsigned buttonToModifier[] = {
+        PlatformEvent::LeftButtonDown,
+        PlatformEvent::MiddleButtonDown,
+        PlatformEvent::RightButtonDown
+    };
+
     switch (e.type) {
     case WebInputEvent::MouseMove:
     case WebInputEvent::MouseLeave:  // synthesize a move event
@@ -121,6 +133,12 @@ PlatformMouseEventBuilder::PlatformMouseEventBuilder(Widget* widget, const WebMo
 
     case WebInputEvent::MouseUp:
         m_type = PlatformEvent::MouseReleased;
+
+        // The MouseEvent spec requires that buttons indicates the state
+        // immediately after the event takes place. To ensure consistency
+        // between platforms here, we explicitly clear the button that is
+        // in the process of being released.
+        m_modifiers &= ~buttonToModifier[e.button];
         break;
 
     default:
@@ -152,6 +170,12 @@ PlatformWheelEventBuilder::PlatformWheelEventBuilder(Widget* widget, const WebMo
         m_modifiers |= PlatformEvent::AltKey;
     if (e.modifiers & WebInputEvent::MetaKey)
         m_modifiers |= PlatformEvent::MetaKey;
+    if (e.modifiers & WebInputEvent::LeftButtonDown)
+        m_modifiers |= PlatformEvent::LeftButtonDown;
+    if (e.modifiers & WebInputEvent::MiddleButtonDown)
+        m_modifiers |= PlatformEvent::MiddleButtonDown;
+    if (e.modifiers & WebInputEvent::RightButtonDown)
+        m_modifiers |= PlatformEvent::RightButtonDown;
 
     m_hasPreciseScrollingDeltas = e.hasPreciseScrollingDeltas;
 #if OS(MACOSX)