This CL refactors the module decoder so that it can process a list of
section buffers instead of one module buffer. This change is needed for
streaming compilation. Streaming compilation may require additional
changes.

This CL introduces the following interface to the module decoder:
StartDecoding -- starts the decoding
DecodeModuleHeader -- decodes the module header
DecodeSection -- decodes the section
FinishDecoding -- finishes the decoding and returns the WasmModule

Aside from the different interface the biggest change to the module
decoder is the introduction of a buffer_offset, which is the offset
of the current section buffer of the module decoder in the module bytes.
This buffer_offset is used to translate from section offsets to module
offsets and back.

Another nice change is that the module decoder does not have a zone
anymore. Instead the zone is stored directly in the WasmModule where
it belongs. Zone ownership is also more obvious now.

R=mtrofin@chromium.org, clemensh@chromium.org

Change-Id: I815d777ec380f4c617c39e828ea0c9746c0bae20
Reviewed-on: https://chromium-review.googlesource.com/505490
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45374}