Report the TTFB of SPDY headers as the time that the BufferedSpdyFramer received header metadata

Currently, we report `recv_first_byte_time` to a SpdyStream as the time
after all headers have been parsed. In the case of large headers that
span multiple packets, this is often incorrect. Use the time that the
BufferedSpdyFramer received the start of the HEADERS packet instead.

Bug: 568024
Change-Id: Ic5549acd380e70c8c073d3f1d6ef9243b31135d8
Reviewed-on: https://chromium-review.googlesource.com/c/1372471
Commit-Queue: Andrew Comminos <acomminos@fb.com>
Reviewed-by: Bence Béky <bnc@chromium.org>
Cr-Commit-Position: refs/heads/master@{#616168}
6 files changed