storage: fix context import
storage: fix context import

Change-Id: I5366a1ab2c45dfca7a788b87b1c2eefcaf5dab7a
Reviewed-on: https://code-review.googlesource.com/18690
Reviewed-by: Jonathan Amsterdam <e3d3698b2ccd5955e4adf250d0785062d0f9018b@google.com>

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