The signature of an imported function is needed to compile a wrapper in
wasm to call the imported function. The signature is stored in a heap
object which is created when the wasm module is compiled. With this CL
we do not use a pointer to the signature in the heap object but instead
copy the signature and then use a pointer to the copy. A pointer into
a heap object causes problems when a GC is happening.

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

Review-Url: https://codereview.chromium.org/2124743002
Cr-Commit-Position: refs/heads/master@{#37527}