Remove FrameFetchContext::GetLoadingTaskRunner

FrameFetchContext::GetLoadingTaskRunner has overwritten
FetchContext::GetLoadingTaskRunner because when it was introduced a
frame-bound task runner stopped working after it is detached while we
wanted to post some tasks even after the frame is detached.

According to [1] the situation has changed and now it's safe to assume
that we can post a task after the frame is detached.

1: https://crrev.com/c7d8259df9b28da303535314e5094963c7a59e5a

Change-Id: I4a09a44242378795aa7e74ee72459d2a18689c3a
Bugs: 914739
Reviewed-on: https://chromium-review.googlesource.com/c/1391648
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Reviewed-by: Hiroshige Hayashizaki <hiroshige@chromium.org>
Reviewed-by: Hajime Hoshi <hajimehoshi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#620627}
3 files changed