Make sure that we call the destructors on all embedded object by
replacing the WasmInterpreterInternals::Delete method by an actual
destructor. This way, the compiler automatically calls destructors on
all embedded objects, in particular the IdentityMap in the CodeMap.

This change also requires to release managed objects *before*
tearing down the heap, because the wasm interpreter, referenced via
Managed<>, contains global handles. When those are destroyed, the
isolate still needs to be intact.

Drive-by: Fix include guard in managed.h.

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

Change-Id: I9a067f037e013c84e4d697a1e913b27c683bb529
Reviewed-on: https://chromium-review.googlesource.com/466187Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44368}