Add a new SpdyStream::Delegate method to handle trailers

This CL adds a new SpdyStream::Delegate method to surface trailers to
Delegate's implementations. Before this CL, receiving trailers will trigger
a spdy protocol error. However, SpdyHttpStream does not implement
this delegate method as of this CL, so trailers are ignored.

BUG=481033

Review URL: https://codereview.chromium.org/1272283003

Cr-Commit-Position: refs/heads/master@{#342859}
13 files changed