Permissions: Avoid poking at the heap after Blink shutdown

Changes serviceConnectionError() to be a no-op if Platform::current()
is null. This can happen because MessageLoop destruction may invoke
the callback after Blink shutdown.

TBRing since this precise change was recommended by haraken@ and I'd
like to make sure the fix lands ASAP.

BUG=650637
TBR=haraken@chromium.org

Review-Url: https://codereview.chromium.org/2446723002
Cr-Commit-Position: refs/heads/master@{#427190}

third_party/WebKit/Source/modules/permissions/Permissions.cpp

@@ -264,6 +264,13 @@ PermissionService* Permissions::getService(ExecutionContext* executionContext) {
 }
 
 void Permissions::serviceConnectionError() {
+  if (!Platform::current()) {
+    // TODO(rockot): Remove this hack once renderer shutdown sequence is fixed.
+    // Note that reaching this code indicates that the MessageLoop has already
+    // been torn down, so it's impossible for any pending reply callbacks on
+    // |m_service| to fire beyond this point anyway.
+    return;
+  }
   m_service.reset();
 }