Fix warnings from gtest under GCC 5 or newer.

Change-Id: I9661f2fe9d315dccae69caa70d929b5d9d93b7db
(cherry picked from commit 4c83fdd3d7)

third_party/googletest/README.libvpx

@@ -12,4 +12,8 @@ failures, various options for running the tests, and XML test report
 generation.
 
 Local Modifications:
-Removed unused declarations of kPathSeparatorString to have warning free build.
+- Removed unused declarations of kPathSeparatorString to have warning
+  free build.
+- Added GTEST_ATTRIBUTE_UNUSED_ to test registering dummies in TEST_P
+  and INSTANTIATE_TEST_CASE_P to remove warnings about unused variables
+  under GCC 5.

third_party/googletest/src/include/gtest/gtest.h

@@ -16960,7 +16960,7 @@ internal::CartesianProductHolder10<Generator1, Generator2, Generator3,
                       GTEST_TEST_CLASS_NAME_(test_case_name, test_name)>()); \
       return 0; \
     } \
-    static int gtest_registering_dummy_; \
+    static int gtest_registering_dummy_ GTEST_ATTRIBUTE_UNUSED_; \
     GTEST_DISALLOW_COPY_AND_ASSIGN_(\
         GTEST_TEST_CLASS_NAME_(test_case_name, test_name)); \
   }; \
@@ -16972,7 +16972,7 @@ internal::CartesianProductHolder10<Generator1, Generator2, Generator3,
 # define INSTANTIATE_TEST_CASE_P(prefix, test_case_name, generator) \
   ::testing::internal::ParamGenerator<test_case_name::ParamType> \
       gtest_##prefix##test_case_name##_EvalGenerator_() { return generator; } \
-  int gtest_##prefix##test_case_name##_dummy_ = \
+  int gtest_##prefix##test_case_name##_dummy_ GTEST_ATTRIBUTE_UNUSED_ = \
       ::testing::UnitTest::GetInstance()->parameterized_test_registry(). \
           GetTestCasePatternHolder<test_case_name>(\
               #test_case_name, __FILE__, __LINE__)->AddTestCaseInstantiation(\