When LEGACY_GL_EMULATION is enabled, we cannot use WebGL VAO extension effectively, since we need to maintain the VAO state in the emulation layer to be able to programmatically construct the appropriate VBOs for rendering. Therefore only use WebGL VAO extension only when LEGACY_GL_EMULATION is not set. (An alternative could be to glGet() the VAO state when bound, or to maintain a shadow cache alongside the VAOs in the emulation layer, but that wholly defeats the performance purposes of VAOs, so it's the same to just never use the VAO extension when emulating). This fixes the browser GL emulation VAO tests.