Refactor Worklet class to use ScriptResource.

Following the recommendations discussed in http://crrev.com/2146633002, this patch switches Worklet.cpp to use ScriptResource to load a worklet script, so it can follow the code path defined in FrameFetchContext::canRequest for CSP checks.

BUG=567358

Review-Url: https://codereview.chromium.org/2178223002
Cr-Commit-Position: refs/heads/master@{#409232}
23 files changed