Check open-device callback before invoking in MediaStreamManager

This avoids invoking a null callback in case a device is opened more
than once due to multiple open requests coming from the same renderer
process.

Bug: 821362
Change-Id: If55189b5f08c230f56925e2f8c810ec22a5ecd84
Reviewed-on: https://chromium-review.googlesource.com/973163Reviewed-by: Olga Sharonova <olka@chromium.org>
Commit-Queue: Guido Urdaneta <guidou@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545023}

content/browser/renderer_host/media/media_stream_manager.cc

@@ -1312,9 +1312,10 @@ void MediaStreamManager::FinalizeRequestFailed(
       break;
     }
     case MEDIA_OPEN_DEVICE_PEPPER_ONLY: {
-      DCHECK(request->open_device_cb);
-      std::move(request->open_device_cb)
-          .Run(false /* success */, std::string(), MediaStreamDevice());
+      if (request->open_device_cb) {
+        std::move(request->open_device_cb)
+            .Run(false /* success */, std::string(), MediaStreamDevice());
+      }
       break;
     }
     case MEDIA_DEVICE_ACCESS: {
@@ -1334,10 +1335,10 @@ void MediaStreamManager::FinalizeRequestFailed(
 void MediaStreamManager::FinalizeOpenDevice(const std::string& label,
                                             DeviceRequest* request) {
   DCHECK_CURRENTLY_ON(BrowserThread::IO);
-  DCHECK(request->open_device_cb);
-
-  std::move(request->open_device_cb)
-      .Run(true /* success */, label, request->devices.front());
+  if (request->open_device_cb) {
+    std::move(request->open_device_cb)
+        .Run(true /* success */, label, request->devices.front());
+  }
 }
 
 void MediaStreamManager::FinalizeMediaAccessRequest(