Replacing EffectivePriority with Priority for QUIC.

Replacing EffectivePriority with Priority for QUIC.

The only place we had changing priority was "if the response headers were
serialized and not sent"  After we moved to having a separate headers stream
we never utilized the QuicWriter's header buffer, instead passing
headers directly to the QUIC layer which serialized them and sent them with
their own special priority.

Removing the idea of mutable priority enables having the write blocked list
track stream priorty, removing SPDY-specific logic from our shared code and
moving us closer to the new H2 style write scheduler logic.

Merge internal change: 107610692

R=rch@chromium.org
BUG=

[Ryan Hamilton, 2015-11-21 22:16:52]

lgtm