Clearing stepping was originally introduced in http://codereview.chromium.org/7889039
But DebugEvaluate now also uses a DisableBreak scope, which makes sure we don't step
inside the evaluated code.

R=yurys@chromium.org
BUG=chromium:467180
LOG=N

Review URL: https://codereview.chromium.org/1129363003

Cr-Commit-Position: refs/heads/master@{#28461}