Fix create OnScreen surface on Mac for compositor_unittests

After issue https://codereview.chromium.org/227473009 there is no code for
create OnScreen surface for Mac which is using in compositor_unittests.
Result:
18 tests crashed:
    LayerWithDelegateTest.ConvertPointToLayer_Medium
    LayerWithDelegateTest.ConvertPointToLayer_Simple
    LayerWithDelegateTest.DelegatedLayer
    LayerWithDelegateTest.SchedulePaintFromOnPaintLayer
    LayerWithDelegateTest.SetBoundsWhenInvisible
    LayerWithNullDelegateTest.SetBoundsSchedulesPaint
    LayerWithNullDelegateTest.Visibility
    LayerWithRealCompositorTest.CompositorObservers
    LayerWithRealCompositorTest.Delegate
    LayerWithRealCompositorTest.Draw
    LayerWithRealCompositorTest.DrawPixels
    LayerWithRealCompositorTest.DrawTree
    LayerWithRealCompositorTest.Hierarchy
    LayerWithRealCompositorTest.HierarchyNoTexture
    LayerWithRealCompositorTest.ModifyHierarchy
    LayerWithRealCompositorTest.Opacity
    LayerWithRealCompositorTest.ScaleReparent
    LayerWithRealCompositorTest.ScaleUpDown

If surface is created then all tests passed.

R=sievers@chromium.org, ccameron1@chromium.org, danakj@chromium.org

TEST=compositor_unittests

Review URL: https://codereview.chromium.org/504933002

Cr-Commit-Position: refs/heads/master@{#291863}

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: