content: Remove non-thread-safe use of WeakPtrs.

content/child/child_thread_impl.cc

Index: content/child/child_thread_impl.cc
diff --git a/content/child/child_thread_impl.cc b/content/child/child_thread_impl.cc
index 41c5543cb695f52c268f9d49724e2813515282ed..c82f7919984240515025f8a1ea767af23b2860a6 100644
--- a/content/child/child_thread_impl.cc
+++ b/content/child/child_thread_impl.cc
@@ -477,6 +477,10 @@ void ChildThreadImpl::Init(const Options& options) {
 }
 
 ChildThreadImpl::~ChildThreadImpl() {
+  // ChildDiscardableSharedMemoryManager has to be destroyed while
+  // |thread_safe_sender_| is still valid.
+  discardable_shared_memory_manager_.reset();
+
 #if defined(OS_ANDROID)
   {
     base::AutoLock lock(g_lazy_child_thread_lock.Get());