In lazy compilation, we only compile one function at a time, and we
might not have the wire bytes of the whole module available.
This CL prepares the WasmCompilationUnit for this setting.
It will also be helpful for streaming compilation.

Also, the ErrorThrower (which might heap-allocate) is not stored in the
WasmCompilationUnit any more. Instead, it is passed to the
FinishCompilation method which is allowed to heap-allocate.

R=titzer@chromium.org, ahaas@chromium.org
BUG=v8:5991

Review-Url: https://codereview.chromium.org/2726553003
Cr-Commit-Position: refs/heads/master@{#43573}