Rename and modify SpdyPriorityTree.

Rename SpdyPriorityTree to Http2PriorityWriteScheduler and move into
http2_write_scheduler.h, to better align with PriorityWriteScheduler, which
handles the equivalent role for SPDY/3. Also change code and comments to refer
to streams instead of nodes.

Modify Http2PriorityWriteScheduler API to be more consistent with
PriorityWriteScheduler. Previously, in cases of API misuse,
Http2PriorityWriteScheduler methods would return false to indicate error, in
turn sometimes requiring other values to be returned via out-params. This CL
makes Http2PriorityWriteScheduler use a style similar to PriorityWriteScheduler,
where invalid method calls trigger DFATAL, and status is no longer reflected by
return value.

Modify Http2PriorityWriteScheduler so that it picks streams one at a time, as
opposed to calculating a full ordering across all streams in a batch. Picking
streams one at a time is necessary to match usage of existing
WriteBlockedList/PriorityWriteScheduler by SPDY and QUIC code. This change also
adds proper round-robining of picks across usable streams with the same
priority.

This CL lands server changes 112594726, 112597015, and 112782384 by mpw.

BUG=488484

Review URL: https://codereview.chromium.org/1660283002

Cr-Commit-Position: refs/heads/master@{#373743}