Revert 113282 - Isolates generic DnsClient from AsyncHostResolver.

Revert 113282 - Isolates generic DnsClient from AsyncHostResolver. 

There were a few memory issues including access of unadressable memory.

DnsClient provides a generic DNS client that allows fetching resource records. 
DnsClient is very lightweight and does not support aggregation, queuing or 
prioritization of requests. 

This is the first CL in a series to merge AsyncHostResolver into 
HostResolverImpl. 

Also introduces general-purpose BigEndianReader/Writer. 

BUG=90881
TEST=./net_unittests


Review URL: http://codereview.chromium.org/8762001

TBR=szym@chromium.org
Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=113384