PlatformThreadHandle: remove public id() interface

Remove PlatformThreadHandle.id() interface.
Also remove id_ member that is not needed any more.

BUG=468793
TEST=./out/Debug/base_unittests
TEST=git cl try

Review URL: https://codereview.chromium.org/1207823004

Cr-Commit-Position: refs/heads/master@{#340242}
16 files changed