Fix ReadingListDistillerPage::HandleGoogleCachedAMPPage crash

Check for main WebFrame before handling a google cached amp page in
reading list distiller.

A WebState may not have a main WebFrame if the page can not execute
JavaScript or if it is navigating.

Fixed: 1329125
Change-Id: Id8bd0d4c3e409f5de7936d51fd45907f0fa40d7d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3669662
Reviewed-by: Sergio Collazos <sczs@chromium.org>
Auto-Submit: Mike Dougherty <michaeldo@chromium.org>
Commit-Queue: Mike Dougherty <michaeldo@chromium.org>
Commit-Queue: Sergio Collazos <sczs@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1007900}
NOKEYCHECK=True
GitOrigin-RevId: e7c4cf72b568fc6ea46a6fd5b8463a0a2451ddb6
1 file changed