Note to crash team: This CL is a refactor and has no intended behavior change.

This change was scripted by https://crbug.com/676387#c8.

Note-worthy for the reviewer:
 * SequenceChecker enforces thread-safety but not thread-affinity!
   If the classes that were updated are thread-affine (use thread local
   storage or a third-party API that does) they should be migrated to
   ThreadChecker instead.
 * ~NonThreadSafe() used to implcitly check in its destructor
   ~Sequence/ThreadChecker() doesn't by design. To keep this CL a
   no-op, an explicit check was added to the destructor of migrated
   classes.
 * NonThreadSafe used to provide access to subclasses, as such
   the |sequence_checker_| member was made protected rather than
   private where necessary.

BUG=676387
This CL was uploaded by git cl split.

R=sergeyu@chromium.org

Review-Url: https://codereview.chromium.org/2911893003
Cr-Commit-Position: refs/heads/master@{#476279}