RenderFrameImpl: simplify CommitNavigation path

- Remove ResetSourceLocation. We can do it locally in
  DocumentLoader::StartLoading instead.
- Move commit call out of LoadDataURL to CommitNavigation.
- Remove some code duplication in CommitNavigation.
- Add weak_this check which was missing in LoadDataURL path.

Bug: 855189
Change-Id: I1d97a6f28ff574d47b0eb2f592705dfcf8daf026
Reviewed-on: https://chromium-review.googlesource.com/c/1352527
Reviewed-by: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Dmitry Gozman <dgozman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#614170}
7 files changed