discardable shared memory: Allow resetting the allocator instance.

base/memory/discardable_memory_allocator.cc

Index: base/memory/discardable_memory_allocator.cc
diff --git a/base/memory/discardable_memory_allocator.cc b/base/memory/discardable_memory_allocator.cc
index 002a3ba5e4763c360dc0a96861d0c53758bdb3bc..ee288ff4b20c7706598bc2004202e952f50add10 100644
--- a/base/memory/discardable_memory_allocator.cc
+++ b/base/memory/discardable_memory_allocator.cc
@@ -16,12 +16,7 @@ DiscardableMemoryAllocator* g_allocator = nullptr;
 // static
 void DiscardableMemoryAllocator::SetInstance(
     DiscardableMemoryAllocator* allocator) {
-  DCHECK(allocator);
-
-  // Make sure this function is only called once before the first call
-  // to GetInstance().
-  DCHECK(!g_allocator);
-
+  DCHECK(!allocator || !g_allocator);
   g_allocator = allocator;
 }