When the instance has imported memory, calling GrowMemory should update the memory object to have a consistent view of the memory. This fixes the failing emscripten test case, added a reduced test that simulates the same behavior.

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

Review-Url: https://chromiumcodereview.appspot.com/2438673006
Cr-Commit-Position: refs/heads/master@{#40490}