Avoid stack overflow by decreasing the height of the trees used in EventsDispatchingInDeeplyNestedShadowTrees.html

I expect that the decreased height of the trees no longer causes a stack overflow.

BUG=352069

Review URL: https://codereview.chromium.org/196793012

git-svn-id: svn://svn.chromium.org/blink/trunk@169231 bbb929c8-8fbe-4397-9dbb-9b2b20218538