Small nav-related user activation cleanups

Found a few small cleanups in this area:

- In LocalFrameClientImpl::BeginNavigation: a ResourceRequest, which is
  already passed in, contains the transient user activation bit, no need
  to pass it in separately. (Note: a WebNavigationInfo actually has two
  copies of this bit because of this fact). The value on both the
  ResourceRequest and the passed in parameter both come from
  LocalFrame::HasTransientUserActivation in FrameLoader::StartNavigation
- Removed the user gesture param in a few places where it was unused
- Some stale TODOs around code not needed after UserActivation v2 ships.

Bug=None

Change-Id: I8ed9de40f241a20816dd20323886ceb2ce83013f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2159771
Commit-Queue: David Bokan <bokan@chromium.org>
Auto-Submit: David Bokan <bokan@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Reviewed-by: Mustaq Ahmed <mustaq@chromium.org>
Reviewed-by: Charlie Harrison <csharrison@chromium.org>
Cr-Commit-Position: refs/heads/master@{#764823}