Make plugin elements return false from canStartSelection.

third_party/WebKit/Source/core/html/HTMLCanvasElement.h

 /*
  * Copyright (C) 2004, 2006, 2009, 2010 Apple Inc. All rights reserved.
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
@@ skipping 194 matching lines @@
 protected:
     void didMoveToNewDocument(Document& oldDocument) override;
 
 private:
     explicit HTMLCanvasElement(Document&);
 
     using ContextFactoryVector = Vector<OwnPtr<CanvasRenderingContextFactory>>;
     static ContextFactoryVector& renderingContextFactories();
     static CanvasRenderingContextFactory* getRenderingContextFactory(int);
 
+    bool canStartSelection() const override { return false; }

[yosin_UTC9, 2016/06/21 08:09:32]

HTMLCanvasElement::canStartSelection() should return true if this canvas shows
fall back content.

Example: You can see fall back content when JavaScript is disabled.

<style>canvas { outline: orange 1px solid; }</style>
<canvas id="canvas" width="300" height="300">
  An alternative text describing what your canvas displays. 
</canvas>

[aelias_OOO_until_Jul13, 2016/06/21 22:07:50]

OK, I removed the canvas part of this patch -- it can be improved in a future
patch if there is a need.  (The canvas fallback mechanism is different and more
complex than plugins, so this is looking like too out-of-scope for this patch.)

+
     void parseAttribute(const QualifiedName&, const AtomicString&, const AtomicString&) override;
     LayoutObject* createLayoutObject(const ComputedStyle&) override;
     bool areAuthorShadowsAllowed() const override { return false; }
 
     void reset();
 
     PassOwnPtr<ImageBufferSurface> createImageBufferSurface(const IntSize& deviceSize, int* msaaSampleCount);
     void createImageBuffer();
     void createImageBufferInternal(PassOwnPtr<ImageBufferSurface> externalSurface);
     bool shouldUseDisplayList(const IntSize& deviceSize);
@@ skipping 27 matching lines @@
 
     mutable RefPtr<Image> m_copiedImage; // FIXME: This is temporary for platforms that have to copy the image buffer to render (and for CSSCanvasValue).
 
     // Used for OffscreenCanvas that controls this HTML canvas element
     OwnPtr<CanvasSurfaceLayerBridge> m_surfaceLayerBridge;
 };
 
 } // namespace blink
 
 #endif // HTMLCanvasElement_h