Add a new priority level, THROTTLED, below IDLE.

THROTTLED is used when the consumer is expecting that low priority
requests will soon be followed by higher priority ones.  It directs the
network stack to reserve resources to handle the coming requests.  In
practice this will mean that only a very small number of THROTTLED requests
may be outstanding at any given time.

BUG=600839

Review-Url: https://chromiumcodereview.appspot.com/1866483002
Cr-Commit-Position: refs/heads/master@{#426831}
diff --git a/net/base/prioritized_dispatcher_unittest.cc b/net/base/prioritized_dispatcher_unittest.cc
index fffb47f..7d0b90e 100644
--- a/net/base/prioritized_dispatcher_unittest.cc
+++ b/net/base/prioritized_dispatcher_unittest.cc
@@ -20,8 +20,10 @@
 
 // We rely on the priority enum values being sequential having starting at 0,
 // and increasing for higher priorities.
-static_assert(MINIMUM_PRIORITY == 0u && MINIMUM_PRIORITY == IDLE &&
-                  IDLE < LOWEST && LOWEST < HIGHEST &&
+static_assert(MINIMUM_PRIORITY == 0u && MINIMUM_PRIORITY == THROTTLED &&
+                  THROTTLED < IDLE &&
+                  IDLE < LOWEST &&
+                  LOWEST < HIGHEST &&
                   HIGHEST <= MAXIMUM_PRIORITY,
               "priority indexes incompatible");