Skip RenderFrameImpl::WillSendRequest for the main request

We don't need it during CommitNavigation - we already
called WillSendRequest during BeginNavigation instead.

This is a preparation to stop (ab)using WebURLRequest when
committing a navigation - the callbacks won't be issued as well.

Bug: 855189
Change-Id: If0b792cd3e43171fe3830005af61c660b93e8c6e
Reviewed-on: https://chromium-review.googlesource.com/c/1271875
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Reviewed-by: Camille Lamy <clamy@chromium.org>
Cr-Commit-Position: refs/heads/master@{#598892}
2 files changed