Fix create OnScreen surface on Mac for compositor_unittests

ui/gl/gl_surface_mac.cc

Index: ui/gl/gl_surface_mac.cc
diff --git a/ui/gl/gl_surface_mac.cc b/ui/gl/gl_surface_mac.cc
index 1852beb928500a099eccb0912bae5bd8f820723a..41f67981a6209e0736340f3dfd39c4be7c1a36c5 100644
--- a/ui/gl/gl_surface_mac.cc
+++ b/ui/gl/gl_surface_mac.cc
@@ -116,6 +116,12 @@ scoped_refptr<GLSurface> GLSurface::CreateViewGLSurface(
       NOTIMPLEMENTED() << "No onscreen support on Mac.";
       return NULL;
     }
+    case kGLImplementationOSMesaGL: {
+      scoped_refptr<GLSurface> surface(new GLSurfaceOSMesaHeadless());
+      if (!surface->Initialize())
+        return NULL;
+      return surface;
+    }
     case kGLImplementationMockGL:
       return new GLSurfaceStub;
     default: