[mparch] Add primary page check to DidFinishNavigation in
ReaderModeIconView.

This CL adds the IsInPrimaryMainFrame() check to
ReaderModeIconView::DidFinishNavigation(). Non-primary page navigation
should not change the ink drop state in the icon. A browser test with
prerendering is also added to cover the case.

Bug: 1218946
Change-Id: I564832f543238c26e709b75b665ed034a665c041
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3372041
Reviewed-by: Kevin McNee <mcnee@chromium.org>
Reviewed-by: Caroline Rising <corising@chromium.org>
Commit-Queue: Hyowon Kim <hyowon@igalia.com>
Cr-Commit-Position: refs/heads/main@{#957406}