Have SendBeacon use ResourceFetcher

This CL makes SendBeacon use the standard way to load resources rather
than hacky PingLoaderImpl.

Bug: 695939
Change-Id: Ice4f09a392337a7920f04f3227177fc8a1681e06
Reviewed-on: https://chromium-review.googlesource.com/541158
Reviewed-by: Mike West <mkwst@chromium.org>
Reviewed-by: Takeshi Yoshino <tyoshino@chromium.org>
Commit-Queue: Yutaka Hirano <yhirano@chromium.org>
Cr-Commit-Position: refs/heads/master@{#482573}
8 files changed