Only use "final" on MSVS2012+, to keep pdfium building with 2010.

BUG=pdfium:26
TBR=bo_xu@foxitsoftware.com

Review URL: https://codereview.chromium.org/418463002

core/include/fxcrt/fx_system.h

diff --git a/core/include/fxcrt/fx_system.h b/core/include/fxcrt/fx_system.h
index a757de4..7488e9d 100644
--- a/core/include/fxcrt/fx_system.h
+++ b/core/include/fxcrt/fx_system.h
@@ -276,7 +276,7 @@
 #ifdef __cplusplus
 };
 
-#if defined(__clang__) || defined(_MSC_VER)
+#if defined(__clang__) || _MSC_VER >= 1700
 #define FX_FINAL final
 #elif defined(__GNUC__) && __cplusplus >= 201103 && \
       (__GNUC__ * 10000 + __GNUC_MINOR__ * 100) >= 40700