[Refactor] Rename DnsResolutionObserver --> HostResolver::Observer.

TEST=this is striclty a refactoring; just needs to compile and pass existing tests.

BUG=14056

Review URL: http://codereview.chromium.org/126110

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@18372 0039d316-1c4b-4281-b951-d872f2087c98
9 files changed