Mojo C++ bindings: fix [D]CHECKs in MultiplexRouter and SyncHandleRegistry.

mojo/public/cpp/bindings/lib/sync_handle_registry.cc

Index: mojo/public/cpp/bindings/lib/sync_handle_registry.cc
diff --git a/mojo/public/cpp/bindings/lib/sync_handle_registry.cc b/mojo/public/cpp/bindings/lib/sync_handle_registry.cc
index cc8313717577821d89b56f0d1c5c78feebcafc26..f556c4c21598b318c1b080cb7857e6c15253342b 100644
--- a/mojo/public/cpp/bindings/lib/sync_handle_registry.cc
+++ b/mojo/public/cpp/bindings/lib/sync_handle_registry.cc
@@ -108,9 +108,17 @@ SyncHandleRegistry::SyncHandleRegistry() {
 SyncHandleRegistry::~SyncHandleRegistry() {
   DCHECK(thread_checker_.CalledOnValidThread());
 
+  // This object may be destructed after the thread local storage slot used by
+  // |g_current_sync_handle_watcher| is reset during thread shutdown.
+  // For example, another slot in the thread local storage holds a referrence to
+  // this object, and that slot is cleaned up after
+  // |g_current_sync_handle_watcher|.
+  if (!g_current_sync_handle_watcher.Pointer()->Get())
+    return;
+
   // If this breaks, it is likely that the global variable is bulit into and
   // accessed from multiple modules.
-  CHECK_EQ(this, g_current_sync_handle_watcher.Pointer()->Get());
+  DCHECK_EQ(this, g_current_sync_handle_watcher.Pointer()->Get());
 
   g_current_sync_handle_watcher.Pointer()->Set(nullptr);
 }