Added google/benchmark submodule.

Author: cmumford

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "third_party/googletest"]
 	path = third_party/googletest
 	url = https://github.com/google/googletest.git
+[submodule "third_party/benchmark"]
+	path = third_party/benchmark
+	url = https://github.com/google/benchmark

CMakeLists.txt

@@ -298,6 +298,8 @@ if(LEVELDB_BUILD_TESTS)
   # This project is tested using GoogleTest.
   add_subdirectory("third_party/googletest")
 
+  add_subdirectory("third_party/benchmark")
+
   # GoogleTest triggers a missing field initializers warning.
   if(LEVELDB_HAVE_NO_MISSING_FIELD_INITIALIZERS)
     set_property(TARGET gtest
@@ -318,7 +320,7 @@ if(LEVELDB_BUILD_TESTS)
 
         "${test_file}"
     )
-    target_link_libraries("${test_target_name}" leveldb gmock gtest)
+    target_link_libraries("${test_target_name}" leveldb gmock gtest benchmark)
     target_compile_definitions("${test_target_name}"
       PRIVATE
         ${LEVELDB_PLATFORM_NAME}=1