At the moment we check only in the module-decoder if the sections in a
module appear at most once. The code section, however, we process
already before this check. With this CL we check that there is at most
one code section before we start processing it.

R=clemensh@chromium.org
TEST=WasmStreamingDecoderTest.TwoCodeSections

Bug: chromium:771916
Change-Id: Icc79d5a87ab39f450a35c688f74ea5e67cae4b3c
Reviewed-on: https://chromium-review.googlesource.com/702379Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#48314}