[wasm] Fix race in LookupNativeModule method.

R=clemensh@chromium.org
TEST=mjsunit/wasm/data-segments
BUG=v8:7424,v8:8009

Change-Id: I6578b77f69d0d56d7f824486acd4c150e4d20ad0
Reviewed-on: https://chromium-review.googlesource.com/1160224Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54859}

src/wasm/wasm-code-manager.cc

@@ -965,6 +965,7 @@ WasmCode* WasmCodeManager::GetCodeFromStartAddress(Address pc) const {
 }
 
 NativeModule* WasmCodeManager::LookupNativeModule(Address pc) const {
+  base::LockGuard<base::Mutex> lock(&native_modules_mutex_);
   if (lookup_map_.empty()) return nullptr;
 
   auto iter = lookup_map_.upper_bound(pc);