Fix GN build on Linux.

BUG=

Review URL: https://codereview.chromium.org/38723002

Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: f12f69dfc640da9e7f3104aa13b08727bd319e37

tools/gn/secondary/third_party/WebKit/Source/wtf/BUILD.gn

diff --git a/tools/gn/secondary/third_party/WebKit/Source/wtf/BUILD.gn b/tools/gn/secondary/third_party/WebKit/Source/wtf/BUILD.gn
index e5e1b6c..da3f26e 100644
--- a/tools/gn/secondary/third_party/WebKit/Source/wtf/BUILD.gn
+++ b/tools/gn/secondary/third_party/WebKit/Source/wtf/BUILD.gn
@@ -25,7 +25,7 @@
   if (is_linux) { # (gcc_version >= 46) {
     # Disable warnings about c++0x compatibility, as some names (such as
     # nullptr) conflict with upcoming c++0x types.
-    cflags_cc += [ "-Wno-c++0x-compat" ]
+    cflags_cc = [ "-Wno-c++0x-compat" ]
   }
 
 #  # Some warnings occur in WTF headers, so they must also be disabled