If incremental GC starts before imports linking, and sees a wasm
function, it won't revisit that after the imports that function are linked.
As a result, the import code objects may be GC-ed. This change
addresses this issue.

BUG=

Review-Url: https://codereview.chromium.org/2113183002
Cr-Commit-Position: refs/heads/master@{#37507}