Simplify base::Timer implementation

Simplify base::Timer implementation

This CL simplifies base::Timer family by:
 - Use default parameter to remove constructor overloads.
 - Move BaseTimerMethodPointer::Start to Timer, so that we can remove
   entire BaseTimerMethodPointer.
 - Converts Timer at DelayedTimer's protected parent to a private member
   of DelayedTimer.

BUG=

[tzik, 2017-05-01 08:56:03]

The CQ bit was checked by tzik@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-05-01 08:56:21]

Dry run: CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[tzik, 2017-05-01 09:14:48]

The CQ bit was checked by tzik@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2017-05-01 09:15:00]

Dry run: CQ is trying da patch.

Follow status at:
https://chromium-cq-status.appspot.com/v2/patch-status/codereview.chromium.or...

[tzik, 2017-05-01 09:44:01]

Description was changed from

==========
Simplify base::Timer implementation

BUG=
==========

to

==========
Simplify base::Timer implementation

This CL simplifies base::Timer family by:
 - Use default parameter to remove constructor overloads.
 - Move BaseTimerMethodPointer::Start to Timer, so that we can remove
   entire BaseTimerMethodPointer.
 - Converts Timer at DelayedTimer's protected parent to a private member
   of DelayedTimer.

BUG=
==========

[commit-bot: I haz the power, 2017-05-01 11:27:03]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2017-05-01 11:27:03]

Dry run: This issue passed the CQ dry run.