Add Plugin.PpapiSyncIPCTime metric.

ppapi/proxy/plugin_dispatcher.cc

Index: ppapi/proxy/plugin_dispatcher.cc
diff --git a/ppapi/proxy/plugin_dispatcher.cc b/ppapi/proxy/plugin_dispatcher.cc
index 0f15645e6b7b77b9b46c5ba30e5afcdffcc170d7..91ac68268ccd01c026398cd759d62dfe2f846ff0 100644
--- a/ppapi/proxy/plugin_dispatcher.cc
+++ b/ppapi/proxy/plugin_dispatcher.cc
@@ -9,6 +9,7 @@
 #include "base/compiler_specific.h"
 #include "base/logging.h"
 #include "base/message_loop/message_loop.h"
+#include "base/metrics/histogram_macros.h"
 #include "base/trace_event/trace_event.h"
 #include "ipc/ipc_message.h"
 #include "ipc/ipc_sync_channel.h"
@@ -206,6 +207,7 @@ bool PluginDispatcher::Send(IPC::Message* msg) {
   if (msg->is_sync()) {
     // Synchronous messages might be re-entrant, so we need to drop the lock.
     ProxyAutoUnlock unlock;
+    SCOPED_UMA_HISTOGRAM_TIMER("Plugin.PpapiSyncIPCTime");
     return SendMessage(msg);
   }
   return SendMessage(msg);