Record forced alternative service histogram.

net/http/http_server_properties_impl.cc

Index: net/http/http_server_properties_impl.cc
diff --git a/net/http/http_server_properties_impl.cc b/net/http/http_server_properties_impl.cc
index baa0edbdf8d2397a2607b09ff80e174cba042ee8..8eb0d69de88f05242567196f133f05ff0b956a78 100644
--- a/net/http/http_server_properties_impl.cc
+++ b/net/http/http_server_properties_impl.cc
@@ -8,6 +8,7 @@
 #include "base/logging.h"
 #include "base/memory/scoped_ptr.h"
 #include "base/message_loop/message_loop.h"
+#include "base/metrics/histogram.h"
 #include "base/stl_util.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
@@ -245,6 +246,8 @@ AlternateProtocolInfo HttpServerPropertiesImpl::GetAlternateProtocol(
       it->second.probability >= alternate_protocol_probability_threshold_)
     return it->second;
 
+  UMA_HISTOGRAM_BOOLEAN("Net.ForceAlternativeService",
+                        g_forced_alternate_protocol != nullptr);
   if (g_forced_alternate_protocol)
     return *g_forced_alternate_protocol;