Make plugin elements return false from canStartSelection.

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

Index: third_party/WebKit/Source/core/html/HTMLCanvasElement.h
diff --git a/third_party/WebKit/Source/core/html/HTMLCanvasElement.h b/third_party/WebKit/Source/core/html/HTMLCanvasElement.h
index 1c72248b08640e274dd43730b87f6a8ede270bc6..2d2af66ff9e7690b4d260d525d81c392ec7c8965 100644
--- a/third_party/WebKit/Source/core/html/HTMLCanvasElement.h
+++ b/third_party/WebKit/Source/core/html/HTMLCanvasElement.h
@@ -212,6 +212,8 @@ private:
     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; }