[WebGL][OpenGLES] Enable MSAA support for WebGL Canvas

[WebGL][OpenGLES] Enable MSAA support for WebGL Canvas
https://bugs.webkit.org/show_bug.cgi?id=130955

Patch by Byungseon Shin <sun.shin@lge.com> on 2014-03-31
Reviewed by Dean Jackson.

To avoid aliasing issues when we render content to WebGL canvas,
 we need to implement MSAA support.
- Imagination OpenGLES GPU Driver already support MSAA, so we
 need a separate code path to enable it.

* platform/graphics/Extensions3D.h:
* platform/graphics/opengl/Extensions3DOpenGLCommon.cpp:
(WebCore::Extensions3DOpenGLCommon::Extensions3DOpenGLCommon):
* platform/graphics/opengl/Extensions3DOpenGLCommon.h:
(WebCore::Extensions3DOpenGLCommon::isImagination):
* platform/graphics/opengl/GraphicsContext3DOpenGLES.cpp:
(WebCore::GraphicsContext3D::reshapeFBOs):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@166563 268f45cc-cd09-0410-ab3c-d52691b4dbfc