Add reprioritization to socket pools.

net/http/http_network_transaction.cc

Index: net/http/http_network_transaction.cc
diff --git a/net/http/http_network_transaction.cc b/net/http/http_network_transaction.cc
index b7ceb430bd9ea8bc3f1b202cdf7aaf483f3b4976..66bd911ca96c5dc112b131a24b31d4adfd63cfec 100644
--- a/net/http/http_network_transaction.cc
+++ b/net/http/http_network_transaction.cc
@@ -421,11 +421,12 @@ void HttpNetworkTransaction::PopulateNetErrorDetails(
 void HttpNetworkTransaction::SetPriority(RequestPriority priority) {
   priority_ = priority;
 
-  // TODO: Note that if throttling is ever implemented below this
-  // level, either of the two below calls may result in request
-  // completion, callbacks, and the potential deletion of this object
-  // (like the call below to throttle_->SetPriority()).  In that case,
-  // this code will need to be modified.
+  // TODO(rdsmith): Note also that if priority-based throttling is
+  // ever implemented below this level, either of the two below calls
+  // may result in request completion, callbacks, and the potential
+  // deletion of this object (like the call below to
+  // throttle_->SetPriority()).  In that case, this code will need to
+  // be modified.

[mmenke, 2017/01/05 20:18:35]

Ok, now I understand what you mean because of your response to me, but I still
think the socket pools implement priority-based throttling.  Once the pool limit
is reached, the group with the highest priority open socket request gets the
next socket slot.

Maybe be more explicit:  "Note that if any level below this ever implements a
throttling mechanism where changing a request's priority may cause *any* request
to synchronously succeed or fail, that callback could synchronously delete
|this|, and this code could potentially crash.  In that case, this code will
need to be modified."

[Randy Smith (Not in Mondays), 2017/01/13 23:05:44]

Done, though I felt a need to be a bit wordier once I got going.  Let me know
what you think.

   if (stream_request_)
     stream_request_->SetPriority(priority);
   if (stream_)