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This CL makes the following changes:

(1) Make SpdySession use Socket::ReadIfReady(). This new behavior is
    hidden behind a field trial flag, SocketReadIfReady.
(2) Make SpdySession lazily allocate |read_buffer| on demand.
(3) Make SpdySession release |read_buffer| when Read() completes or
    when ReadIfReady() cannot be completed synchronously.

Re-allocating an 8KiB buffer should be cheap, see more discussion
at https://groups.google.com/a/chromium.org/d/msg/project-trim/QtliUsApxyk/AOSnyuY7CgAJ

BUG=690915

Review-Url: https://codereview.chromium.org/2734933002
Cr-Commit-Position: refs/heads/master@{#455770}