[PartitionAlloc] Don't restore the last error around TLS access, if not needed

Amendment to crrev.com/c/2529150 to avoid a call to SetLastError in
a typical case.

Bug: 998048
Change-Id: Id39f072da22017e10882d57fa32c61882265478a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2537741Reviewed-by: Benoit L <lizeb@chromium.org>
Reviewed-by: Yuki Shiino <yukishiino@chromium.org>
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Cr-Commit-Position: refs/heads/master@{#827795}

base/allocator/partition_allocator/partition_tls.h

@@ -61,7 +61,9 @@ ALWAYS_INLINE void* PartitionTlsGet(PartitionTlsKey key) {
   // when it succeeds."
   DWORD saved_error = GetLastError();
   void* ret = TlsGetValue(key);
-  SetLastError(saved_error);
+  // Only non-zero errors need to be restored.
+  if (UNLIKELY(saved_error))
+    SetLastError(saved_error);
   return ret;
 }