Similar to WasmInterpreter::Thread, we now also use the pimpl idiom for
InterpretedFrame, hiding the implementation completely in the .cc file.
This allows us to store just two things per InterpretedFrameImpl: The
corresponding thread, and the frame index.
The external interface changes to always return a std::unique_ptr,
because the object layout is not known via the public interface, hence
objects cannot be stack allocated. They also cannot be copied or passed
by value.

The frame inspection interface will be tested after another fix in
https://chromium-review.googlesource.com/474749.

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

Change-Id: I7b109da73df745fac97ec72cb0cf4f0ad71e5da9
Reviewed-on: https://chromium-review.googlesource.com/472887Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44600}