Initial implementation of bindings and basic classes for worklets.

https://drafts.css-houdini.org/worklets/

WorkerScriptController renamed to GlobalScopeScriptController to share
code with WorkletScriptController.

Additional super-class (AbstractGlobalScope) created for
WorkerGlobalScope and WorkletGlobalScope to share ExecutionContext
related implementation.

Notable pieces left to implement:
 - support for Worklet::importScript
 - devtools integration
 - off-main-thread implementation for compositorworklet & audioworklet

Intent to implement: https://groups.google.com/a/chromium.org/d/msg/blink-dev/kl8ct3ub3a8/-WfzBTkCAgAJ

BUG=567358

Review URL: https://codereview.chromium.org/1535943005

Cr-Commit-Position: refs/heads/master@{#369323}
24 files changed