Names passed for imports and exports are checked during decoding,
leading to errors if they are no valid UTF-8. Function names are not
checked during decode, but rather lead to undefined being returned at
runtime if they are not UTF-8.

We need to do these checks on the Wasm side, since the factory
methods assume to get valid UTF-8 strings.

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

Review-Url: https://codereview.chromium.org/1967023004
Cr-Commit-Position: refs/heads/master@{#36208}