Introduce blink::Script

This CL introduces blink::Script and ClassicScript that correspond to
https://html.spec.whatwg.org/#concept-script and
https://html.spec.whatwg.org/#classic-script, respectively.

This CL moves classic-script-only logic in ScriptLoader::DoExecuteScript()
(AccessControlStatus and MIME type check) to ClassicScript.

In the rest of this CL, we replace ScriptSourceCode with Script and
wrap ScriptSourceCode by ClassicScript where needed.

Although this CL makes ScriptLoader to handle blink::Script, currently
module scripts are not actually processed because all the call sites supply
ClassicScript, and thus this CL shouldn't change the behavior.

BUG=594639, 686281

Review-Url: https://codereview.chromium.org/2780463002
Cr-Commit-Position: refs/heads/master@{#464161}
13 files changed