[simulator] Fix missing lock guard for I-cache flush.

R=clemensh@chromium.org
BUG=v8:7456

Change-Id: I5b7c151d375267a6a8a24c611b8a13fe38619656
Reviewed-on: https://chromium-review.googlesource.com/924036Reviewed-by: Clemens Hammacher <clemensh@chromium.org>
Commit-Queue: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51369}

src/simulator-base.cc

@@ -74,6 +74,7 @@ void* SimulatorBase::RedirectExternalReference(void* external_function,
 Redirection::Redirection(void* external_function, ExternalReference::Type type)
     : external_function_(external_function), type_(type), next_(nullptr) {
   next_ = Simulator::redirection();
+  base::LockGuard<base::Mutex> lock_guard(Simulator::i_cache_mutex());
   Simulator::SetRedirectInstruction(
       reinterpret_cast<Instruction*>(address_of_instruction()));
   Simulator::FlushICache(Simulator::i_cache(),