Scheduler/child/TimeSource could be replaced with base/time/DefaultTickClock.

They both are totally same and TimeSource is removed.

BUG=494892
R=skyostil@chromium.org, mithro@mithis.com

Review URL: https://codereview.chromium.org/1163143002

Cr-Commit-Position: refs/heads/master@{#333035}
TestTimeSource::TestTimeSource(scoped_refptr<cc::TestNowSource> time_source)
    : time_source_(time_source) {
}

TestTimeSource::TestTimeSource(scoped_refptr<cc::TestNowSource> time_source)
    : time_source_(time_source) {
}
