For correct error positions, the FunctionBody struct stored a {base}
pointer to the beginning of the wasm module bytes, in addition to the
{start} and {end} pointer of the function body within the module bytes.
For streaming compilation, we do not have all module bytes in a single
chunk of memory. Therefore this CL changes the FunctionBody such that it
does not store the base pointer but the offset of the function body
within the module. I did the same change already some time ago for the
{Decoder}.

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

Change-Id: I5138fbe270d0f5166a7dcc5cb8f3fe78a298bff6
Reviewed-on: https://chromium-review.googlesource.com/544863Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Andreas Haas <ahaas@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46170}