PromiseRejectionEvent: Update comment to match what the code is doing.

Since e1c54ad2 ("Update
PromiseRejectionEvent IDL to match latest spec") from more than 2 years ago
the code is returning undefined instead of null. Make the comment match the
code to avoid confusion.

TBR=yukishiino,haraken

No-Try: True
Change-Id: Ie931aef81fbe7510ccbf7b1c0e3753b32a64e948
Reviewed-on: https://chromium-review.googlesource.com/894384
Commit-Queue: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Raphael Kubo da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/master@{#533246}

third_party/WebKit/Source/core/events/PromiseRejectionEvent.cpp

@@ -42,8 +42,8 @@ ScriptPromise PromiseRejectionEvent::promise(ScriptState* script_state) const {
 }
 
 ScriptValue PromiseRejectionEvent::reason(ScriptState* script_state) const {
-  // Return null when the value is accessed by a different world than the world
-  // that created the value.
+  // Return undefined when the value is accessed by a different world than the
+  // world that created the value.
   if (reason_.IsEmpty() || !CanBeDispatchedInWorld(script_state->World()))
     return ScriptValue(script_state, v8::Undefined(script_state->GetIsolate()));
   return ScriptValue(script_state,