DNS: Support reprioritization of requests.

net/dns/host_resolver_mojo.cc

Index: net/dns/host_resolver_mojo.cc
diff --git a/net/dns/host_resolver_mojo.cc b/net/dns/host_resolver_mojo.cc
index 1daf6338fd425bffd0ad1db3a6497b3f5d164325..d65a81e1cde63a5b0bd88b5e5727cfc18a0f2532 100644
--- a/net/dns/host_resolver_mojo.cc
+++ b/net/dns/host_resolver_mojo.cc
@@ -92,6 +92,11 @@ int HostResolverMojo::ResolveFromCache(const RequestInfo& info,
   return ResolveFromCacheInternal(info, CacheKeyForRequest(info), addresses);
 }
 
+void HostResolverMojo::ChangeRequestPriority(RequestHandle req,
+                                             RequestPriority priority) {
+  // Do nothing, since Resolve() discarded the priority anyway.

[Randy Smith (Not in Mondays), 2016/04/28 18:04:40]

Oh, for goodness sake.  What uses the DNS resolver through the mojo interface? 
(This is not a blocking comment on this CL, I'm just a bit miffed at mojoization
tossing priority.)

[Julia Tuttle, 2016/05/04 14:43:20]

No idea!

[mmenke, 2016/05/04 17:33:16]

The out of process PAC stuff, I would assume.  I believe the PAC code has always
tossed priority, so this most likely inherited the behavior from that.

Could be completely wrong about all that, though.

[Randy Smith (Not in Mondays), 2016/05/05 00:07:25]

Matt's probably right.  Willing to put a TODO in the Mojo code near the
declaration of Resolve() to *not* drop priority on the floor?  If it's not
needed for the current use case, that's fine, but I don't want someone else
starting to rely on that implementation and not realize they don't get priority.

[Julia Tuttle, 2016/05/05 12:47:56]

I put a note in the .h instead, where I think a user might be more likely to see
it.

[mmenke, 2016/05/05 12:51:12]

I think there's actually a bug filed against the proxy to correctly send the
priorities, too...Anyhow, agree that this interface should support it, too.

+}
+
 void HostResolverMojo::CancelRequest(RequestHandle req) {
   DCHECK(thread_checker_.CalledOnValidThread());
   // Deleting the Job closes the HostResolverRequestClient connection,