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After this change, users can define a repeated field in protobuf,
message Message {
  repeated <type> foo;
}

and use
    GetParamSize(sizer, p.foo())
    WriteParam(m, p.foo())
and
    ReadParam(m, iter, r->mutable_foo())
to serialize and deserialize the field.

Serializing and deserializing a RepeatedField / RepeatedPtrField are needed in
change https://codereview.chromium.org/2964613002/.

BUG=650926

Review-Url: https://codereview.chromium.org/2968003005
Cr-Commit-Position: refs/heads/master@{#487593}