[mahi] MahiWebContentManager <=> MahiManager communication

This CL makes MahiManager and MahiWebContentManager communicate
directly because now we don't need to support Lacros.

To make "#include"s eligible, this CL adds a new header of
MahiWebContentManager in components path, and make the original
mahi_web_contents_manager.h/cc the real impl of it.

And fixes tests accordingly.

Bug: b:356035887
Change-Id: I7aaab53dc1655c0ed78d2d2b5dfed2b3a6ab8a55
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5898452
Reviewed-by: Hidehiko Abe <hidehiko@chromium.org>
Commit-Queue: Xinglong Luan <alanlxl@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1365453}