This adapts the debugger so that the first break event starting the
stepping process can come from optimized code. TurboFan supports a
debugger statement and hence can be the top-most frame whenever the
Debug::HandleDebugBreak handler is triggered.

R=yangguo@chromium.org
TEST=mjsunit/debug,cctest/test-debug

Review URL: https://codereview.chromium.org/1038613002

Cr-Commit-Position: refs/heads/master@{#27447}