* introduced DebugInterface::PrepareStep and DebugInterface::ClearStepping method.
Inspector calls these methods only on pause and not interseted in calling this for not current break_id so we don't need to expose debug interface with break_id argument and can only check that current break_id is valid.

BUG=chromium:652939,v8:5510
R=yangguo@chromium.org,dgozman@chromium.org

Review-Url: https://chromiumcodereview.appspot.com/2423153002
Cr-Commit-Position: refs/heads/master@{#40450}