Rename spdy_priority_tree*

Rename spdy_priority_tree*.  This CL is solely so that
https://crrev.com/1660283002 can be rebased on it, and thus version history
is not completely lost.

This CL lands part of server change 112594726 by mpw.

BUG=488484

Review URL: https://codereview.chromium.org/1668773003

Cr-Commit-Position: refs/heads/master@{#373607}
3 files changed