Use the new hotword extension in the launcher when it is enabled.

chrome/browser/search/hotword_service.cc

Index: chrome/browser/search/hotword_service.cc
diff --git a/chrome/browser/search/hotword_service.cc b/chrome/browser/search/hotword_service.cc
index e29a08ff4fa424dd9e944fc2a24f96fd567e1172..2f75b0394f5b73731f80cbde6ea0afd001deb541 100644
--- a/chrome/browser/search/hotword_service.cc
+++ b/chrome/browser/search/hotword_service.cc
@@ -4,6 +4,7 @@
 
 #include "chrome/browser/search/hotword_service.h"
 
+#include "base/command_line.h"
 #include "base/i18n/case_conversion.h"
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram.h"
@@ -20,6 +21,7 @@
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/search/hotword_service_factory.h"
 #include "chrome/common/chrome_paths.h"
+#include "chrome/common/chrome_switches.h"
 #include "chrome/common/extensions/extension_constants.h"
 #include "chrome/common/pref_names.h"
 #include "content/public/browser/browser_thread.h"
@@ -173,6 +175,12 @@ bool HotwordService::DoesHotwordSupportLanguage(Profile* profile) {
   return false;
 }
 
+// static
+bool HotwordService::IsExperimentalHotwordingEnabled() {
+  CommandLine* command_line = CommandLine::ForCurrentProcess();
+  return command_line->HasSwitch(switches::kEnableExperimentalHotwording);
+}
+
 HotwordService::HotwordService(Profile* profile)
     : profile_(profile),
       extension_registry_observer_(this),
@@ -447,7 +455,7 @@ void HotwordService::OnHotwordSearchEnabledChanged(
 }
 
 void HotwordService::RequestHotwordSession(HotwordClient* client) {
-  if (!IsServiceAvailable() || client_)
+  if (!IsServiceAvailable() || (client_ && client_ != client))

[arv (Not doing code reviews), 2014/08/14 15:48:06]

This looks suspicious. Why are we exiting if the passed in client is different
from client_? Why are we assigning the client_ below? Why are we passing in
client?

[Jun Mukai, 2014/08/14 22:10:08]

IIRC there is only one client instance right now. Do we add them?  As arv points
out, the current code of client handling does not care so much about the
scenario of multiple clients.

[Anand Mistry (off Chromium), 2014/08/15 00:14:51]

I have no plans to add additional clients. My only requirement is that an
existing client be able to start another session since the current one will be
implicitly stopped when the hotword is triggered. Maybe I need a "resume" method
instead, but that seems unnecessary.

[Jun Mukai, 2014/08/15 00:30:07]

You seems not answering to our concerns.  If we can still assume single client,
why do you have to add the condition of client_ != client?

     return;
 
   client_ = client;