[iOS] Disable Reading mode entry points if the page is ineligible

This CL adds a `CurrentPageSupportsReaderMode()` method to the Reader
mode tab helper which is used to disable entry points when the current
page is ineligible to Reader mode.

Bug: 416226085
Change-Id: Ie21db5004c739c5dc1b0ed3ccab38a0fbc7dbf4e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6532419
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Commit-Queue: Quentin Pubert <qpubert@google.com>
Reviewed-by: Nohemi Fernandez <fernandex@google.com>
Cr-Commit-Position: refs/heads/main@{#1460560}