Unskip test debugger-step-into-custom-element-callbacks.html.

Seems to pass w/o any significant changes.

BUG=381102
TBR=loislo, yurys

Review URL: https://codereview.chromium.org/334633007

git-svn-id: svn://svn.chromium.org/blink/trunk@176616 bbb929c8-8fbe-4397-9dbb-9b2b20218538