Steps to reproduce:
1. Open the attached project (863530-TestCase)
2. Open the Assets/Test.unity scene
3. Select the Game view
4. Enable gizmos option in the Game view
4. Observe that in the Game view both cameras render all objects icons and gizmos

Note:
CamerA renders layer A
CamerB renders layer B
CubeA set to layer A
CubeB set to layer B
GizmoA set to layer A
GizmoB set to layer B

Expected result: Gizmos and Icons should be masked by camera with layer culling mask
Actual result: Gizmos and Icons are not masked by camera with layer culling mask

Reproduced with: 5.3.7p3, 5.4.3p4, 5.5.0f3, 5.5.0p3, 5.6.0b3