All debugging-related information is now stored inside a dedicated
object, which is only allocated if debugging support is needed.
This is also where later a reference to the interpreter will be stored
for executing to-be-debugged functions and providing stack inspection.

R=titzer@chromium.org, ahaas@chromium.org
BUG=chromium:613110

Review-Url: https://codereview.chromium.org/2050953003
Cr-Commit-Position: refs/heads/master@{#37055}