DataSaver support for off-main-thread-fetch

This CL gets the DataSaver setting on the main thread and pass it to
WebWorkerFetchContext (WorkerFetchContextImpl, ServiceWorkerFetchContextImpl).
WorkerFetchContext::AddAdditionalRequestHeaders() will check the flag on the
worker thread and set "Save-Data" header if enabled.

BUG=443374,722774

Review-Url: https://codereview.chromium.org/2886663002
Cr-Commit-Position: refs/heads/master@{#472288}
12 files changed