base: Minor MemoryDumpProvider cleanup.

Leave it up to the implementation of this interface to determine
if assignment and copy should be disallowed.

BUG=

Review URL: https://codereview.chromium.org/1107403003

Cr-Commit-Position: refs/heads/master@{#327492}

base/trace_event/memory_dump_provider.h

@@ -27,7 +27,6 @@ class BASE_EXPORT MemoryDumpProvider {
   MemoryDumpProvider() {}
   virtual ~MemoryDumpProvider() {}
 
- private:
   DISALLOW_COPY_AND_ASSIGN(MemoryDumpProvider);
 };