WebLayer: remove NavigationCallback.onReadyToCommitNavigation

The implementation side and the aidl are left alone in case it
comes in handy in the future. Thus, only the public API is
affected.

Bug: 1174193
Change-Id: I62ea9a5886e932b1469c380416d947f2a276c00e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2674525


Reviewed-by: Scott Violet <sky@chromium.org>
Commit-Queue: Evan Stade <estade@chromium.org>
Cr-Commit-Position: refs/heads/master@{#851130}