Fix warnings from gtest under GCC 5 or newer.

Change-Id: I9661f2fe9d315dccae69caa70d929b5d9d93b7db
(cherry picked from commit 4c83fdd3d7cacd66a2c64451db0fb64e5b2ecf48)

third_party/googletest/README.libvpx

diff --git a/third_party/googletest/README.libvpx b/third_party/googletest/README.libvpx
index 7201a67..1eca78d 100644
--- a/third_party/googletest/README.libvpx
+++ b/third_party/googletest/README.libvpx
@@ -12,4 +12,8 @@
 generation.
 
 Local Modifications:
-Removed unused declarations of kPathSeparatorString to have warning free build.
\ No newline at end of file
+- 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.
\ No newline at end of file

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

diff --git a/third_party/googletest/src/include/gtest/gtest.h b/third_party/googletest/src/include/gtest/gtest.h
index 4f3804f..581a44e 100644
--- a/third_party/googletest/src/include/gtest/gtest.h
+++ b/third_party/googletest/src/include/gtest/gtest.h
@@ -16960,7 +16960,7 @@
                       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 @@
 # 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(\