Prolong ResourceFetcher's lifetime for keepalive loaders

This CL has ResourceFetcher have a SelfKeepAlive member to extend its
lifetime when it has keepalive loaders. Previously ResourceLoader had
the member, but this CL removes it.

This CL also introduces a hard timeout for keepalive loaders.

Bug: 695939
Change-Id: I19cda1522b2884d8fb07a3f623513b7fa576ce7b
Reviewed-on: https://chromium-review.googlesource.com/609763
Reviewed-by: Takeshi Yoshino <tyoshino@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#495476}
4 files changed