Add ExecutionContext and ScriptState support to blink::TaskRunnerHelper

Many of users of TaskRunnerHelper have ExecutionContext or ScriptState
rather than Document or Frame, which are a few step away from EC or SS.
This CL add overloads of TaskRunnerHelper::getUnthrottledTaskRunner for
EC and SS for shorthand.

BUG=624696

Review-Url: https://codereview.chromium.org/2178263002
Cr-Commit-Position: refs/heads/master@{#407721}
2 files changed