Vulkan: Expose the unimplemented GL_EXT_occlusion_query_boolean extension. Chrome needs this extension to be exposed to load the query entry points, even for emulated query types. BUG=angleproject:2855 Change-Id: I09dbb45c391df64d67858f9131c45b164947a707 Reviewed-on: https://chromium-review.googlesource.com/1255507 Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org> Commit-Queue: Geoff Lang <geofflang@chromium.org>