[iOS] Dismiss Reader mode chip after transition to small entrypoint

When the Reader mode contextual chip transitions to a small entrypoint
while the page action menu entrypoint is available for the current tab,
it should be replaced by the page action menu entrypoint. This CL adds
code to notify a contextual panel configuration item that it completed
a transition to a small entrypoint, and ensures that the Reader mode
chip invalidates itself when that happens.

Bug: 432213672
Change-Id: I8c04bdc93afd5d1d97c99ebff400ed2cc9d07f51
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6764674
Auto-Submit: Quentin Pubert <qpubert@google.com>
Reviewed-by: Guillaume Jenkins <gujen@google.com>
Reviewed-by: Robbie Gibson <rkgibson@google.com>
Commit-Queue: Quentin Pubert <qpubert@google.com>
Cr-Commit-Position: refs/heads/main@{#1491323}