storage: add WRITER role constant
storage: add WRITER role constant

Fixes #504.

Change-Id: Ia0d14bf7a2ab279a7e60f7d2ae8b29937c07a836
Reviewed-on: https://code-review.googlesource.com/10750
Reviewed-by: Jason Hall <54dc689b89cd1edd3b8cc82d44c3d9f567e86e1f@google.com>
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go