shill: Convert code to use the newest version of libchrome.

The biggest change is a switch from using the deprecated
Task and CallbackN mechanisms to using the new Callback
mechanism.

Note: Original CL was https://gerrit.chromium.org/gerrit/16156.
This is logically another patch to that CL, but since the
latter was already merged, and is considered closed by
Gerrit, it's necessary to create a new CL.

BUG=chromium-os:15330
TEST=Build shill and run it on a zgb with a modem. Build and
run unit tests.
CQ-DEPEND=I37628863370323d30cac493764ea28f8ffd42637

Change-Id: I3ae78a3aa44ec167b79f2170d07650ece888254f
Reviewed-on: https://gerrit.chromium.org/gerrit/18030
Reviewed-by: Eric Shienbrood <ers@chromium.org>
Tested-by: Eric Shienbrood <ers@chromium.org>
Commit-Ready: Eric Shienbrood <ers@chromium.org>
diff --git a/ip_address.cc b/ip_address.cc
index 5281d85..a6ae127 100644
--- a/ip_address.cc
+++ b/ip_address.cc
@@ -62,7 +62,7 @@
 }
 
 // static
-int IPAddress::GetPrefixLengthFromMask(Family family, const string &mask) {
+size_t IPAddress::GetPrefixLengthFromMask(Family family, const string &mask) {
   switch (family) {
     case kFamilyIPv4: {
       in_addr_t mask_val = inet_network(mask.c_str());