[wasm] Missing lock in import wrapper cache

Add a lock to prevent races between {WasmImportWrapperCache::Get} and
the cache modification scope.

R=clemensb@chromium.org

Change-Id: Ife281c127c765d3ca57f58c975e15a76833983ca
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1965588
Commit-Queue: Thibaud Michaud <thibaudm@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65458}

src/wasm/wasm-import-wrapper-cache.cc

@@ -25,6 +25,7 @@ WasmCode*& WasmImportWrapperCache::operator[](
 
 WasmCode* WasmImportWrapperCache::Get(compiler::WasmImportCallKind kind,
                                       FunctionSig* sig) const {
+  base::MutexGuard lock(&mutex_);
   auto it = entry_map_.find({kind, sig});
   DCHECK(it != entry_map_.end());
   return it->second;

src/wasm/wasm-import-wrapper-cache.h

@@ -49,14 +49,14 @@ class WasmImportWrapperCache {
   // cache.
   V8_EXPORT_PRIVATE WasmCode*& operator[](const CacheKey& key);
 
-  // Assumes the key exists in the map.
+  // Thread-safe. Assumes the key exists in the map.
   V8_EXPORT_PRIVATE WasmCode* Get(compiler::WasmImportCallKind kind,
                                   FunctionSig* sig) const;
 
   ~WasmImportWrapperCache();
 
  private:
-  base::Mutex mutex_;
+  mutable base::Mutex mutex_;
   std::unordered_map<CacheKey, WasmCode*, CacheKeyHash> entry_map_;
 };