Restrict BackGestureContextualNudgeDelegate to the primary frame tree

The automatic rewrite of `IsInPrimaryMainFrame` done at
https://crrev.com/c/2957195 wasn't working for subframes, which need to
also be restricted to the primary frame tree.

This CL also fixes a DCHECK when `HasSubframeNavigationEntryCommitted`
is called for a non-primary main frame navigation.

Bug: 1218946
Change-Id: I8d0c54c40ef0c54829174fbeba1b7c09d817ffbb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3568426


Reviewed-by: Xiyuan Xia <xiyuan@chromium.org>
Commit-Queue: Henrique Ferreiro <hferreiro@igalia.com>
Reviewed-by: Kevin McNee <mcnee@chromium.org>
Cr-Commit-Position: refs/heads/main@{#999809}