Before, the encoded variant was stored in the compiled module, and the
decoded one in the debug info (per instance).
The decoded table was a FixedArray of ByteArrays.
Now, also the decoded table is a flat ByteArray, and it encodes whether
it is encoded or decoded. This saves memory and allows to store encoded
and decoded variant in the same field. The table is automatically
decoded on the first use.

This CL also removes some unused and unimplemented methods from
WasmDebugInfo (probably merge artifacts). That class is now pretty much
empty, but we might still need it for breakpoint support.

R=titzer@chromium.org, ahaas@chromium.org

Review-Url: https://codereview.chromium.org/2522953002
Cr-Commit-Position: refs/heads/master@{#41316}