Remove FetchContext::DispatchNetworkQuiet

The only caller, IdlenessDetector::WillProcessTask, has an access to
the DocumentLoader, so we don't need the FetchContext method.

Bug: 914739
Change-Id: Iedbe27b5992e114d507075584c9d3fd287de19ab
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1538059
Reviewed-by: Hiroki Nakagawa <nhiroki@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#644178}