Check delegate before running Pepper media-device enumeration/monitoring requests.

content/renderer/pepper/pepper_device_enumeration_host_helper.cc

Index: content/renderer/pepper/pepper_device_enumeration_host_helper.cc
diff --git a/content/renderer/pepper/pepper_device_enumeration_host_helper.cc b/content/renderer/pepper/pepper_device_enumeration_host_helper.cc
index 2fd9a3b36d184db03fc2ff2a99c74408fab14305..dc4967c11216fcc587aac13517b450cbadd3f28c 100644
--- a/content/renderer/pepper/pepper_device_enumeration_host_helper.cc
+++ b/content/renderer/pepper/pepper_device_enumeration_host_helper.cc
@@ -35,6 +35,9 @@ class PepperDeviceEnumerationHostHelper::ScopedEnumerationRequest
     if (!owner->document_url_.is_valid())
       return;
 
+    if (!owner->delegate_)

[yzshen1, 2017/05/05 17:08:11]

Sorry this has been too long ago. I don't remember this code. :)

Do we understand why delegate could be null? Is there a legitimate reason?

If we early return from, the callback will never be called, does the caller
expect that?

[Guido Urdaneta, 2017/05/05 17:18:12]

It is also expected if the document_url is not valid a few lines above, so I
thought it was OK for a somewhat similar situation of a null delegate. 
I don't know why the delegate can become null. I am familiar with the device
enumeration code, but not so much with the lifetimes of the Pepper objects. I
glanced through the source code, but did not find an obvious reason why this
could be the case.

An alternative to leaving the callback hanging could be to invoke the callback
with an empty result.

I can prepare a patch that does that if you think that is better.

[yzshen1, 2017/05/05 17:22:31]

That makes sense. :)
This is what I am a little concerned about. There may be something wrong
elsewhere leading to an inconsistent state. 
You definitely know this code better than I do now. So it is up to you. :)

+      return;
+
     requested_ = true;
 
     // Note that the callback passed into
@@ -44,7 +47,6 @@ class PepperDeviceEnumerationHostHelper::ScopedEnumerationRequest
     // EnumerateDevicesCallbackBody() to ensure that we always call |callback|
     // asynchronously.
     sync_call_ = true;
-    DCHECK(owner->delegate_);
     owner->delegate_->EnumerateDevices(
         owner->device_type_, owner->document_url_,
         base::Bind(&ScopedEnumerationRequest::EnumerateDevicesCallbackBody,
@@ -91,9 +93,11 @@ class PepperDeviceEnumerationHostHelper::ScopedMonitoringRequest
     if (!owner_->document_url_.is_valid())
       return;
 
+    if (!owner->delegate_)
+      return;
+
     requested_ = true;
 
-    DCHECK(owner_->delegate_);
     // |callback| is never called synchronously by StartMonitoringDevices(),
     // so it is OK to pass it directly, even if |callback| destroys |this|.
     subscription_id_ = owner_->delegate_->StartMonitoringDevices(