base: Cleanup: Simplify the declaration of Lock class.

This is the first step into merging Lock and LockImpl. The later will be
merged into Lock when things get more clearer.

BUG=283054
TEST=base_unittests --gtest_filter=LockTest.*
R=darin@chromium.org, brettw@chromium.org

Review URL: https://codereview.chromium.org/159283003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252864 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252864