Mark classes with final dtors as final

- -Wfinal-dtor-non-final-class warns on classes with final dtors but
  not final classes.
- Error messages are better when the class is marked final.
- Fix existing issues in code base and remove warning exemption

Bug: 999886
Test: no errors building
Change-Id: I85c8b03f3dab773e5fb017acdcf0b6682f1c9e01
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2466978
Commit-Queue: Zentaro Kavanagh <zentaro@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816687}

base/task/sequence_manager/sequence_manager_impl.cc

@@ -142,7 +142,8 @@ char* PrependHexAddress(char* output, const void* address) {
 
 }  // namespace
 
-class SequenceManagerImpl::NativeWorkHandleImpl : public NativeWorkHandle {
+class SequenceManagerImpl::NativeWorkHandleImpl final
+    : public NativeWorkHandle {
  public:
   NativeWorkHandleImpl(SequenceManagerImpl* sequence_manager,
                        TaskQueue::QueuePriority priority)

base/task/sequence_manager/task_queue_impl.h

@@ -282,7 +282,7 @@ class BASE_EXPORT TaskQueueImpl {
     TaskQueueImpl* const outer_;
   };
 
-  class TaskRunner : public SingleThreadTaskRunner {
+  class TaskRunner final : public SingleThreadTaskRunner {
    public:
     explicit TaskRunner(scoped_refptr<GuardedTaskPoster> task_poster,
                         scoped_refptr<AssociatedThreadId> associated_thread,