Add support to create memory allocator dump in base::DiscardableMemory

base/test/test_discardable_memory_allocator.cc

Index: base/test/test_discardable_memory_allocator.cc
diff --git a/base/test/test_discardable_memory_allocator.cc b/base/test/test_discardable_memory_allocator.cc
index 23d529c324d74677ca0bba6ed37d94b606ea3aff..8e512d5768cf963ec173acdb5bfe18fea05982bb 100644
--- a/base/test/test_discardable_memory_allocator.cc
+++ b/base/test/test_discardable_memory_allocator.cc
@@ -20,6 +20,12 @@ class DiscardableMemoryImpl : public DiscardableMemory {
   void Unlock() override {}
   void* data() const override { return data_.get(); }
 
+  trace_event::MemoryAllocatorDump* CreateMemoryAllocatorDump(
+      const char* name,
+      trace_event::ProcessMemoryDump* pmd) override {
+    return nullptr;
+  }
+
  private:
   scoped_ptr<uint8_t[]> data_;
 };