[Underscore Migration] Migrate ChildTargetManager
[Underscore Migration] Migrate ChildTargetManager

This patch is part of the underscore migration that adds the private
keyword to all properties that start with an underscore and removes the
underscore.

Bug:chromium:1207342
Change-Id: I1c954db382c9be19ed3f2e0bdb8cd494c671445e
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3074600
Commit-Queue: Jan Scheffler <8abd9e3d93041d096604cb2a1b3070dd7075ba7b@chromium.org>
Commit-Queue: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>
Auto-Submit: Jan Scheffler <8abd9e3d93041d096604cb2a1b3070dd7075ba7b@chromium.org>
Reviewed-by: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>

ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend,ChromeDevTools/devtools-frontend