[iOS] Show Reading mode options when Reading mode chip is tapped

This CL sets the primary action of the Reading mode chip (displayed when
Reading mode is active) to opening Reading mode options.

Bug: 409941529
Change-Id: I5e32ef132a232cbbe35f9ef3222efa1bb7c5d4d9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6678415
Auto-Submit: Quentin Pubert <qpubert@google.com>
Reviewed-by: Nohemi Fernandez <fernandex@google.com>
Commit-Queue: Quentin Pubert <qpubert@google.com>
Cr-Commit-Position: refs/heads/main@{#1479725}