Unify HTTP/2 and SPDY stream schedulers under common WriteScheduler interface.

This is to make it possible for SpdyDispatcher to maintain a single set of
codepaths for dealing with stream scheduling, with differences in scheduling
schemes hidden behind the WriteScheduler interface.  The actual wiring of
WriteScheduler into SpdyDispatcher will be in a follow-on CL.

This CL lands server change 115262424 by mpw.

BUG=488484

Review-Url: https://codereview.chromium.org/1900403004
Cr-Commit-Position: refs/heads/master@{#393758}
diff --git a/net/net.gypi b/net/net.gypi
index 0a571d1..5ac20b6 100644
--- a/net/net.gypi
+++ b/net/net.gypi
@@ -1150,6 +1150,8 @@
       'spdy/spdy_write_queue.cc',
       'spdy/spdy_write_queue.h',
       'spdy/write_blocked_list.h',
+      'spdy/write_scheduler.h',
+      'spdy/write_scheduler.cc',
       'ssl/client_cert_store.h',
       'ssl/client_cert_store_mac.cc',
       'ssl/client_cert_store_mac.h',
@@ -1749,6 +1751,7 @@
       'spdy/spdy_test_utils.h',
       'spdy/spdy_write_queue_unittest.cc',
       'spdy/write_blocked_list_test.cc',
+      'spdy/write_scheduler_test.cc',
       'ssl/channel_id_service_unittest.cc',
       'ssl/client_cert_store_mac_unittest.cc',
       'ssl/client_cert_store_nss_unittest.cc',