storage: use pointer receiver for ObjectAttrs
storage: use pointer receiver for ObjectAttrs

Change-Id: Ifc32cc1a3cd2df361f1c5a42abca514a2c4824e9
Reviewed-on: https://code-review.googlesource.com/8996
Reviewed-by: Michael Darakananda <176c4818757ce942f02114959cee75d35f2575d9@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go