[TreesInViz] Add LayerImpl::hit_test_opaqueness_ to mojo interface.

Otherwise hit tests do not work correctly in TreesInViz mode.

This fixes a bunch of hit test related content_browsertests.

TEST=trees_in_viz bot's content_browsertests

Bug: 410617908
Change-Id: I8bfa2394498230614063a180935f7aa20870f0a5
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6460114
Auto-Submit: Zhenyao Mo <zmo@chromium.org>
Reviewed-by: Alex Gough <ajgo@chromium.org>
Reviewed-by: Vasiliy Telezhnikov <vasilyt@chromium.org>
Commit-Queue: Vasiliy Telezhnikov <vasilyt@chromium.org>
Reviewed-by: Victor Miura <vmiura@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1448327}