[wasm] Add a TODO for reader-writer lock on signature map.

R=clemensh@chromium.org

Bug: 
Change-Id: I5e4cd8dcb9c22cd261b48598c021f84ad712ba58
Reviewed-on: https://chromium-review.googlesource.com/586249Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#46895}

src/wasm/signature-map.h

@@ -37,6 +37,7 @@ class V8_EXPORT_PRIVATE SignatureMap {
     bool operator()(FunctionSig* a, FunctionSig* b) const;
   };
   uint32_t next_ = 0;
+  // TODO(wasm): performance-critical, replace with a reader-writer lock
   std::unique_ptr<base::Mutex> mutex_;
   std::map<FunctionSig*, uint32_t, CompareFunctionSigs> map_;