Worklet: Move common worklet code from modules/worklet to core/workers

After ThreadedWorklet is introduced, worklet code gets more tied up with
worker code, so it'd be reasonable that they are located in the same directory.

BUG=567358

Review-Url: https://codereview.chromium.org/2262483003
Cr-Commit-Position: refs/heads/master@{#413413}
15 files changed