Add support for ProxyResolverErrorObserver to ProxyResolverMojo.

net/BUILD.gn

Index: net/BUILD.gn
diff --git a/net/BUILD.gn b/net/BUILD.gn
index b4b378879f5d9f6e5982df774bf9e28853a812a9..ef7ed9db02f9b275677373eec6323e77b65e5a2c 100644
--- a/net/BUILD.gn
+++ b/net/BUILD.gn
@@ -839,6 +839,8 @@ if (use_v8_in_net && !is_android) {
       "proxy/mojo_proxy_resolver_factory_impl.h",
       "proxy/mojo_proxy_resolver_impl.cc",
       "proxy/mojo_proxy_resolver_impl.h",
+      "proxy/proxy_resolver_error_observer_mojo.cc",
+      "proxy/proxy_resolver_error_observer_mojo.h",
     ]
 
     deps = [
@@ -1493,6 +1495,7 @@ if (!is_android && !is_mac) {
         "proxy/load_state_change_coalescer_unittest.cc",
         "proxy/mojo_proxy_resolver_factory_impl_unittest.cc",
         "proxy/mojo_proxy_resolver_impl_unittest.cc",
+        "proxy/proxy_resolver_error_observer_mojo_unittest.cc",
         "proxy/proxy_resolver_mojo_unittest.cc",
         "proxy/proxy_service_mojo_unittest.cc",
       ]