[cpu-profiler] Protect StringsStorage::GetCopy with a lock

Ensure that we don't concurrently modify the StringsStorage map when
getting a copy of a string.

Bug: v8:11054
Change-Id: I6ad61838d7c5e8a6e9ff21aac04da8d353e41ad5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2648821
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Commit-Queue: Andrew Comminos <acomminos@fb.com>
Cr-Commit-Position: refs/heads/master@{#72416}

src/profiler/strings-storage.cc

@@ -27,6 +27,7 @@ StringsStorage::~StringsStorage() {
 }
 
 const char* StringsStorage::GetCopy(const char* src) {
+  base::MutexGuard guard(&mutex_);
   int len = static_cast<int>(strlen(src));
   base::HashMap::Entry* entry = GetEntry(src, len);
   if (entry->value == nullptr) {