Use macros for SequenceChecker docs.

docs/threading_and_tasks.md

 # Threading and Tasks in Chrome
 
 [TOC]
 
 ## Overview
 
 ### Threads
 
 Every Chrome process has
 
@@ skipping 171 matching lines @@
 
 Usage of locks is discouraged in Chrome. Sequences inherently provide
 thread-safety. Prefer classes that are always accessed from the same sequence to
 managing your own thread-safety with locks.
 
 ```cpp
 class A {
  public:
   A() {
     // Do not require accesses to be on the creation sequence.
-    sequence_checker_.DetachFromSequence();
+    DETACH_FROM_SEQUENCE(sequence_checker_);
   }
 
   void AddValue(int v) {
     // Check that all accesses are on the same sequence.
-    DCHECK(sequence_checker_.CalledOnValidSequence());
+    DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
     values_.push_back(v);
 }
 
  private:
-  base::SequenceChecker sequence_checker_;
+  SEQUENCE_CHECKER(sequence_checker_);
 
   // No lock required, because all accesses are on the
   // same sequence.
   std::vector<int> values_;
 };
 
 A a;
 scoped_refptr<SequencedTaskRunner> task_runner_for_a = ...;
 task_runner->PostTask(FROM_HERE,
                       base::BindOnce(&A::AddValue, base::Unretained(&a)));
@@ skipping 447 matching lines @@
 
  private:
   scoped_refptr<base::SequenceTaskRunner> background_task_runner_ =
       base::CreateSequenceTaskRunnerWithTraits(
           {base::MayBlock(), base::TaskPriority::BACKGROUND});
 }
 ```
 
 Note that this still allows removing all layers of plumbing between //chrome and
 that component since unit tests will use the leaf layer directly.