Use ResourceRequestInfo::GetWebContents in DownloadRequestLimiter

This CL makes DownloadRequestLimiter use a the callback returned by
ResourceRequestInfo::GetWebContents instead of using RFH + RPH IDs. This allows
most download browser_tests to work with PlzNavigate enabled.

BUG=504347

Review URL: https://codereview.chromium.org/1467563002

Cr-Commit-Position: refs/heads/master@{#362699}
15 files changed