EasyUnlock: Only do Bluetooth detection for chromeos.

Bluetooth detection on Mac triggers 500ms adapter polling and causes
performance regression. Since Easy unlock is only offered on ChromeOS
at the moment, disable Bluetooth detection for other platforms while
the polling problem is investigated.

BUG=404482,399067

Review URL: https://codereview.chromium.org/498453002

Cr-Commit-Position: refs/heads/master@{#291409}
git-svn-id: svn://svn.chromium.org/chrome/trunk/src@291409 0039d316-1c4b-4281-b951-d872f2087c98

chrome/browser/signin/easy_unlock_service.cc

@@ -290,7 +290,15 @@ void EasyUnlockService::Initialize() {
       base::Bind(&EasyUnlockService::OnPrefsChanged, base::Unretained(this)));
   OnPrefsChanged();
 
+#if defined(OS_CHROMEOS)
+  // Only start Bluetooth detection for ChromeOS since the feature is
+  // only offered on ChromeOS. Enabling this on non-ChromeOS platforms
+  // previously introduced a performance regression: http://crbug.com/404482
+  // Make sure not to reintroduce a performance regression if re-enabling on
+  // additional platforms.
+  // TODO(xiyuan): Revisit when non-chromeos platforms are supported.
   bluetooth_detector_->Initialize();
+#endif  // defined(OS_CHROMEOS)
 }
 
 void EasyUnlockService::LoadApp() {