Cherry pick r516 from upstream protobuf

Fix compilation for arm64.

TBR=cpu@chromium.org
R=mark@chromium.org
BUG=314720

Review URL: https://codereview.chromium.org/163723003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@251345 0039d316-1c4b-4281-b951-d872f2087c98

third_party/protobuf/README.chromium

@@ -28,6 +28,7 @@ details, and r179806 for the patch.
 
 Revision 504 was cherry-picked from upstream.
 Revision 512 was cherry-picked from upstream.
+Revision 516 was cherry-picked from upstream.
 Revision 517 was cherry-picked from upstream.
 
 Notes about Java:

third_party/protobuf/src/google/protobuf/stubs/platform_macros.h

@@ -49,6 +49,9 @@
 #elif defined(__ARMEL__)
 #define GOOGLE_PROTOBUF_ARCH_ARM 1
 #define GOOGLE_PROTOBUF_ARCH_32_BIT 1
+#elif defined(__aarch64__)
+#define GOOGLE_PROTOBUF_ARCH_AARCH64 1
+#define GOOGLE_PROTOBUF_ARCH_64_BIT 1
 #elif defined(__MIPSEL__)
 #define GOOGLE_PROTOBUF_ARCH_MIPS 1
 #define GOOGLE_PROTOBUF_ARCH_32_BIT 1