Ship keepalive on Fetch API

Intent to ship thread: https://groups.google.com/a/chromium.org/forum/#!topic/Blink-dev/8vqcABTnDF4

Bug: 695939
Change-Id: I917909f4ee0e5cf2967ac4ed44b137e9a9b6e2a1
Reviewed-on: https://chromium-review.googlesource.com/923442
Reviewed-by: Yoav Weiss <yoav@yoav.ws>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#537564}
5 files changed