Added device count test.

chrome/browser/devtools/device/devtools_android_bridge.h

Index: chrome/browser/devtools/device/devtools_android_bridge.h
diff --git a/chrome/browser/devtools/device/devtools_android_bridge.h b/chrome/browser/devtools/device/devtools_android_bridge.h
index d35f8e1c91b166f5e1f370cf88243b1da2417951..1f81ef1f30c4bf1110a5ef74472c29c45ca34c9b 100644
--- a/chrome/browser/devtools/device/devtools_android_bridge.h
+++ b/chrome/browser/devtools/device/devtools_android_bridge.h
@@ -242,6 +242,10 @@ class DevToolsAndroidBridge
 
   static bool HasDevToolsWindow(const std::string& agent_id);
 
+  void set_polling_interval_for_test(int value) {
+    polling_interval_for_test_ = value;
+  }
+
  private:
   friend struct content::BrowserThread::DeleteOnThread<
       content::BrowserThread::UI>;
@@ -293,6 +297,8 @@ class DevToolsAndroidBridge
 
   AndroidDeviceManager::DeviceProviders device_providers_;
   PrefChangeRegistrar pref_change_registrar_;
+
+  int polling_interval_for_test_;
   DISALLOW_COPY_AND_ASSIGN(DevToolsAndroidBridge);
 };