This CL makes the interpreter reentrant by allowing different
activations to be live at the same time. The wasm interpreter keeps a
list of activations and stores the stack height at the start of each
activation. This information is used to unwind just one activation, or
show the right portion of the interpreter stack for each interpreter
entry frame.
The WasmDebugInfo object stores a mapping from frame pointer (of the
interpreter entry) to the activation id in order to identify the
activation based on the physical interpreter entry frame.

R=titzer@chromium.org, ahaas@chromium.org
BUG=v8:5822

Change-Id: Ibbf93f077f907213173a92e0a2f7f3556515e8eb
Reviewed-on: https://chromium-review.googlesource.com/453958
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#43976}