Pass ScriptState directly to TaskRunnerHelper::get()

TaskRunnerHelper::get() has a overload taking ScriptState* instead of
ExecutionContext*, so we should let it handle ScriptState* rather than
getting ExecutionContext* from ScriptState* at callers.

BUG=625927

Review-Url: https://codereview.chromium.org/2701153002
Cr-Commit-Position: refs/heads/master@{#451618}