We emulate break by callling breakProgramCallback function in debugger context, we can just use HandleDebugBreak.
It allows us to move all stepping logic to debug.cc later and remove one usage of debugger context.
+ two minor issues fixed, see tests.

BUG=v8:5510
R=yangguo@chromium.org

Review-Url: https://codereview.chromium.org/2738503006
Cr-Commit-Position: refs/heads/master@{#43750}