According to the spec, section names must be valid UTF-8. This CL adds
a check for that.
Imported and exported names were already checked before.
In order to use the {consume_string} function from the
WasmSectionIterator, it moved it out of the ModuleDecoder into the
anonymous namespace. It now also gets a name for the string to be
parsed, for better error messages.

R=rossberg@chromium.org

Change-Id: I20b1ddb0bd1c7ada237d8303951073310fe1c714
Reviewed-on: https://chromium-review.googlesource.com/470207
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Andreas Rossberg <rossberg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#44550}