SPDY code cleanup. No intended functional change.

SPDY code cleanup. No intended functional change.

Remove the StreamId field from SpdyHeadersHandlerInterface methods. The caller
of those methods should validate StreamId before calling
SpdyHeadersHandlerInterface methods. This change makes
SpdyHeadersHandlerInterface compatible with HpackDecoder. HPACK has no notion of
stream ID, so the decoder cannot call e.g. OnHeader() with a stream ID.

Also replaces some CHECKs with LOG(DFATAL)s.

This lands server changes 83639565 by birenroy and 84396743 by bnc.

BUG=345769

Review URL: https://codereview.chromium.org/851163005

Cr-Commit-Position: refs/heads/master@{#312514}