Remove incorrect DCHECK at the top of FrameLoader::DidFinishNavigation

Bug: 1106608
Change-Id: Ie71fcdea1186761fd6cc7f32ebf38ee0e497d843
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2316695


Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Auto-Submit: Nate Chapin <japhet@chromium.org>
Reviewed-by: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791484}