Fix clang style warnings in webrtc/base

Mostly this consists of marking functions with override when
applicable, and moving function bodies from .h to .cc files.

Not inlining virtual functions with simple bodies such as

  { return false; }

strikes me as probably losing more in readability than we gain in
binary size and compilation time, but I guess it's just like any other
case where enabling a generally good warning forces us to write
slightly worse code in a couple of places.

BUG=163
R=kjellander@webrtc.org, tommi@webrtc.org

Review URL: https://webrtc-codereview.appspot.com/47429004

Cr-Commit-Position: refs/heads/master@{#8656}
git-svn-id: http://webrtc.googlecode.com/svn/trunk@8656 4adac7df-926f-26a2-2b94-8c16560cd09d
diff --git a/webrtc/base/nethelpers.cc b/webrtc/base/nethelpers.cc
index 5d4802d..0c7cce6 100644
--- a/webrtc/base/nethelpers.cc
+++ b/webrtc/base/nethelpers.cc
@@ -60,6 +60,8 @@
 AsyncResolver::AsyncResolver() : error_(-1) {
 }
 
+AsyncResolver::~AsyncResolver() = default;
+
 void AsyncResolver::Start(const SocketAddress& addr) {
   addr_ = addr;
   // SignalThred Start will kickoff the resolve process.
@@ -80,6 +82,14 @@
   return false;
 }
 
+int AsyncResolver::GetError() const {
+  return error_;
+}
+
+void AsyncResolver::Destroy(bool wait) {
+  SignalThread::Destroy(wait);
+}
+
 void AsyncResolver::DoWork() {
   error_ = ResolveHostname(addr_.hostname().c_str(), addr_.family(),
                            &addresses_);