Add API tests for the chrome.hid API.

device/hid/hid_service.cc

Index: device/hid/hid_service.cc
diff --git a/device/hid/hid_service.cc b/device/hid/hid_service.cc
index d2ccaa379f0c6fa3a67588a2380b5d2274236303..071b3968bcd7fdcd21079e58c5226695eb84d72a 100644
--- a/device/hid/hid_service.cc
+++ b/device/hid/hid_service.cc
@@ -55,7 +55,7 @@ HidService* HidService::GetInstance(
 #elif defined(OS_WIN)
     g_service = new HidServiceWin();
 #endif
-    if (g_service != NULL) {
+    if (g_service != nullptr) {
       Destroyer* destroyer = new Destroyer(g_service);
       base::MessageLoop::current()->AddDestructionObserver(destroyer);
     }
@@ -63,6 +63,13 @@ HidService* HidService::GetInstance(
   return g_service;
 }
 
+void HidService::SetInstanceForTest(HidService* instance) {
+  DCHECK(!g_service);
+  g_service = instance;
+  Destroyer* destroyer = new Destroyer(g_service);
+  base::MessageLoop::current()->AddDestructionObserver(destroyer);
+}
+
 HidService::~HidService() {
   DCHECK(thread_checker_.CalledOnValidThread());
 }