GLContextNSView should not assume concrete type GLSurface is GLSurfaceNSView

GLContextNSView should not assume concrete type GLSurface is GLSurfaceNSView

This is follow-up from review http://codereview.chromium.org/8486020.
This eliminates down-cast of GLSurface in CLContextNSView::Initialize in favor of
GLSurfaceNSView::OnMakeCurrent implementation.  Also, allows offscreen path in USE_AURA
due to its usage in SharedResourcesCC.

BUG=104390
TEST=compositor_unittests --gtest_filter=LayerWithRealCompositorTest.*
R=apatrick@chromium.org

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=112231

[Ken Russell (switch to Gerrit), 2011-11-30 00:39:01]

LGTM

http://codereview.chromium.org/8687016/diff/1/ui/gfx/gl/gl_surface_nsview.mm
File ui/gfx/gl/gl_surface_nsview.mm (right):

http://codereview.chromium.org/8687016/diff/1/ui/gfx/gl/gl_surface_nsview.mm#...
ui/gfx/gl/gl_surface_nsview.mm:43: context_ = static_cast<GLContextNSView
*>(context);
Given the other cleanup in this CL, the downcast is unfortunate. This is the
first platform that has different context types for different surfaces.
Presumably MakeCurrent should do some validation that you're using compatible
contexts and surfaces. Anyway, this is a cleanup for the future.