Split OSMesa implementations of *GLContext into GLContextOSMesa and *GLSurfaceOSMesa.

Surfaces are independent of contexts in GL. To facilitate sharing of surfaces between processes, I have separated them from the notion of contexts because contexts cannot be shared between processes.

I started with EGL in r81512 and WGL in r81807. This is the same thing for OSMesa.

GLContextOSMesa still has a pointer to a surface and still has some surface specific operations that just forward through to it. Once I have refactored all the GLContext implementations in this way, I will remove these pointers and the surface specific opertations.

There will not be "view" and "offscreen" GL contexts. Rather there will be a single context type for each backend which can be made current with a surface that directs output either to a view or offscreen surface.

TEST=WebGL locally, try
BUG=none
Review URL: http://codereview.chromium.org/6864031

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@81998 0039d316-1c4b-4281-b951-d872f2087c98

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