Fix lifetime of leveldb::MojoEnv instances.

components/leveldb/leveldb_mojo_proxy.h

Index: components/leveldb/leveldb_mojo_proxy.h
diff --git a/components/leveldb/leveldb_mojo_proxy.h b/components/leveldb/leveldb_mojo_proxy.h
index 2eef1a41d1db0bd2f5a04886e1195cde7b97ef7f..748f194066bbd7d33695a74dd134b6dab9362031 100644
--- a/components/leveldb/leveldb_mojo_proxy.h
+++ b/components/leveldb/leveldb_mojo_proxy.h
@@ -37,6 +37,10 @@ class LevelDBMojoProxy : public base::RefCountedThreadSafe<LevelDBMojoProxy> {
   explicit LevelDBMojoProxy(
       scoped_refptr<base::SingleThreadTaskRunner> task_runner);
 
+  base::SingleThreadTaskRunner* task_runner() const {
+    return task_runner_.get();
+  }
+
   // A private struct to hide the underlying file that holds the lock from our
   // callers, forcing them to go through our LockFile()/UnlockFile() interface
   // so that they don't try to use the underlying pointer from an unsafe thread.