[TreeOutline] small CSS changes
[TreeOutline] small CSS changes

Some small style changes I need for the object viewer to bring the tree
outline closer to how the existing one looks indent-wise.

Bug: chromium:1166669
Change-Id: I4d3990ed4895425cbe60248eebc16e080f4ae48b
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/2715247
Reviewed-by: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>
Commit-Queue: Tim van der Lippe <dba8716ee7f8d16236046f74d2167cb94410f6ed@chromium.org>
Auto-Submit: Jack Franklin <993fcadce4d04090da2fefd557a0995e7966c8d5@chromium.org>

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