Remove FetchContext::LoadingTaskRunner

This is a preliminary refactoring to implement keepalive fetch operations. In
order to set an appropriate task runner for a url loader, we will need the
request information.

This CL
 - Moves SetLoadingTaskRunner call to the creation timing of the WebURLLoader,
 - Removes FetchContext::LoadingTaskRunner as the task runner for a loader
   can depend on the request, and
 - Provides a request parameter to FetchContext::CreateURLLoader.

Bug: 695939
Change-Id: I822392da57428b71dc4a3f50cf1b96c40d4a0315
Reviewed-on: https://chromium-review.googlesource.com/517447
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Kinuko Yasuda <kinuko@chromium.org>
Cr-Commit-Position: refs/heads/master@{#475330}
17 files changed