[wasm] Fix possible gc-ing of wasm code objects

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}

src/wasm/wasm-module.cc

@@ -372,7 +372,8 @@ bool LinkFunction(Handle<Code> unlinked,
         Handle<Code> new_target = code_targets[index];
         if (target != *new_target) {
           it.rinfo()->set_target_address(new_target->instruction_start(),
-                                         SKIP_WRITE_BARRIER, SKIP_ICACHE_FLUSH);
+                                         UPDATE_WRITE_BARRIER,
+                                         SKIP_ICACHE_FLUSH);
           modified = true;
         }
       }