[Cronet iOS] Add base::StatisticsRecorder::IsActive() check

components/cronet/ios/cronet_environment.mm

Index: components/cronet/ios/cronet_environment.mm
diff --git a/components/cronet/ios/cronet_environment.mm b/components/cronet/ios/cronet_environment.mm
index e886659100504e8358719afbb5a616d6e7fdd65d..a48e6834fec52e4583ad6bfe28029c161efbb001 100644
--- a/components/cronet/ios/cronet_environment.mm
+++ b/components/cronet/ios/cronet_environment.mm
@@ -349,7 +349,7 @@ std::string CronetEnvironment::user_agent() {
 }
 
 std::vector<uint8_t> CronetEnvironment::GetHistogramDeltas() {
-  base::StatisticsRecorder::Initialize();
+  DCHECK(base::StatisticsRecorder::IsActive());

[mef, 2017/04/06 22:08:11]

What would happen if they call this on background thread before init completes?

[xunjieli, 2017/04/06 23:58:01]

Is there an entry point  (if not CronetEnvironment::Initialize()) which
guarantees to complete before GetHistogramDeltas() is called?
On Android, we are doing StatisticsRecorder::Initialize() in JNIOnload() which
guarantees to be called before the user could do
CronetEngine.GetHistogramDeltas().

   std::vector<uint8_t> data;
   if (!HistogramManager::GetInstance()->GetDeltas(&data))
     return std::vector<uint8_t>();