Remove unused StyleChangeReason::Plugin.

Source/core/dom/StyleChangeReason.cpp

 // Copyright 2014 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 "config.h"
 #include "core/dom/StyleChangeReason.h"
 
 #include "platform/TraceEvent.h"
 #include "wtf/MainThread.h"
 #include "wtf/StaticConstructors.h"
 
 namespace blink {
 
 namespace StyleChangeReason {
 const char ActiveStylesheetsUpdate[] = "ActiveStylesheetsUpdate";
 const char Animation[] = "Animation";
 const char Attribute[] = "Attribute";
 const char ControlValue[] = "ControlValue";
 const char Control[] = "Control";
 const char DesignMode[] = "DesignMode";
 const char Drag[] = "Drag";
 const char FontSizeChange[] = "FontSizeChange";
 const char Fonts[] = "Fonts";
 const char FullScreen[] = "FullScreen";
 const char Inline[] = "Inline";
 const char Inspector[] = "Inspector";
 const char Language[] = "Language";
 const char LinkColorChange[] = "LinkColorChange";
 const char PlatformColorChange[] = "PlatformColorChange";
-const char Plugin[] = "Plugin";
 const char PropagateInheritChangeToDistributedNodes[] = "PropagateInheritChangeToDistributedNodes";
 const char PseudoClass[] = "PseudoClass";
 const char SVGContainerSizeChange[] = "SVGContainerSizeChange";
 const char SVGCursor[] = "SVGCursor";
 const char SVGFilterLayerUpdate[] = "SVGFilterLayerUpdate";
 const char Shadow[] = "Shadow";
 const char SiblingSelector[] = "SiblingSelector";
 const char StyleInvalidator[] = "StyleInvalidator";
 const char StyleSheetChange[] = "StyleSheetChange";
 const char Validate[] = "Validate";
@@ skipping 20 matching lines @@
     new (NotNull, (void*)&Disabled) AtomicString(":disabled", AtomicString::ConstructFromLiteral);
     new (NotNull, (void*)&Focus) AtomicString(":focus", AtomicString::ConstructFromLiteral);
     new (NotNull, (void*)&Hover) AtomicString(":hover", AtomicString::ConstructFromLiteral);
     new (NotNull, (void*)&Invalid) AtomicString(":invalid", AtomicString::ConstructFromLiteral);
     new (NotNull, (void*)&Unresolved) AtomicString(":unresolved", AtomicString::ConstructFromLiteral);
 }
 
 } // namespace StyleChangeExtraData
 
 } // namespace blink