Throttle url download requests from a RenderFrame

RenderFrameImpl::DownloadUrl() can be abused to trigger a large number
of download to overwhelm the browser process.
This CL rate limits the number of download to a maximum of 10 per second.

Bug: 860045
Change-Id: I490ae3a99eb59aef709b9f1e6be5eb0f787354f4
Reviewed-on: https://chromium-review.googlesource.com/1132485
Commit-Queue: Min Qin <qinmin@chromium.org>
Reviewed-by: John Abd-El-Malek <jam@chromium.org>
Cr-Commit-Position: refs/heads/master@{#574078}
3 files changed