Add helper functions to get per-frame task runners

This CL introduces TaskRunnerHelper that collects helper functions to get task runners.

This CL removes code to look up task runners of m_importsController and m_contextDocument
(I don't think it's needed; we can just fall back to the default task runner
when document->frame() is not available).

BUG=624689

Review-Url: https://codereview.chromium.org/2141873002
Cr-Commit-Position: refs/heads/master@{#404946}
8 files changed