Make base::{Thread,Sequence}CheckerDoNothing noncopyable

This CL makes base::{Thread,Sequence}CheckerDoNothing noncopyable, so
that they are consistent to base::{Thread,Sequence}CheckerImpl, and an
accidental copy of SequenceChecker causes a compile error even on
DCHCEK-disabled build.

Change-Id: I3ac37d87302712ab3decf693d056376c0c2abb91
Reviewed-on: https://chromium-review.googlesource.com/578751
Commit-Queue: Taiju Tsuiki <tzik@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#488614}

base/sequence_checker.h

@@ -67,9 +67,12 @@ namespace base {
 // above macros) to get the right version for your build configuration.
 class SequenceCheckerDoNothing {
  public:
+  SequenceCheckerDoNothing() = default;
   bool CalledOnValidSequence() const WARN_UNUSED_RESULT { return true; }
-
   void DetachFromSequence() {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(SequenceCheckerDoNothing);
 };
 
 #if DCHECK_IS_ON()

base/threading/thread_checker.h

@@ -78,11 +78,12 @@ namespace base {
 // macros) to get the right version for your build configuration.
 class ThreadCheckerDoNothing {
  public:
-  bool CalledOnValidThread() const WARN_UNUSED_RESULT {
-    return true;
-  }
-
+  ThreadCheckerDoNothing() = default;
+  bool CalledOnValidThread() const WARN_UNUSED_RESULT { return true; }
   void DetachFromThread() {}
+
+ private:
+  DISALLOW_COPY_AND_ASSIGN(ThreadCheckerDoNothing);
 };
 
 // Note that ThreadCheckerImpl::CalledOnValidThread() returns false when called