Remove unused caller argument from ContentUiEventHandler

The ContentUiEventHandler instance was being passed as a JNI argument
to its native methods, but was not being used. This commit removes
this redundant argument, simplifying the JNI boundary.

Bug: 417078839
Change-Id: I7e8b8255d9087952cd216cb8cd880913c38c59c3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6687333
Commit-Queue: Rakina Zata Amni <rakina@chromium.org>
Auto-Submit: Keigo Oka <oka@chromium.org>
Reviewed-by: Rakina Zata Amni <rakina@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1480952}