Disable ChromeBrowserProxyResolver behind a flag.

In Brillo and Android targets, there isn't a Chrome instance running
to resolve the proxies for a given URL. While this is not a fatal error
it logs abundant error messages on each request, confusing developers.

This patch compiles the ChromeBrowserProxyResolver only when the
USE_LIBCROS flag is set. This is now disabled by default in Brillo and
Android, but enabled in Chrome OS.

Bug: 24277309
TEST=`mma` on edison-eng. Applied an update, no more proxy error logs.

Change-Id: I27779572bf2fc810cb7846d63e123643d0386b0a
diff --git a/update_attempter.cc b/update_attempter.cc
index aeb433b..b67fcb3 100644
--- a/update_attempter.cc
+++ b/update_attempter.cc
@@ -125,7 +125,9 @@
     : processor_(new ActionProcessor()),
       system_state_(system_state),
       cert_checker_(cert_checker),
+#if USE_LIBCROS
       chrome_proxy_resolver_(libcros_proxy),
+#endif  // USE_LIBCROS
       debugd_proxy_(debugd_proxy) {
 }
 
@@ -155,7 +157,9 @@
   else
     status_ = UpdateStatus::IDLE;
 
+#if USE_LIBCROS
   chrome_proxy_resolver_.Init();
+#endif  // USE_LIBCROS
 }
 
 void UpdateAttempter::ScheduleUpdates() {