Added "-Wno-signed-enum-bitfield" flag to not trigger Clang warning

skia/BUILD.gn

Index: skia/BUILD.gn
diff --git a/skia/BUILD.gn b/skia/BUILD.gn
index e74662610d45975e6f6aa6b0fadf7186271bb99a..e527d063f33a9b322d725b5decadf2a68f62bc3a 100644
--- a/skia/BUILD.gn
+++ b/skia/BUILD.gn
@@ -481,6 +481,11 @@ component("skia") {
     # SkFontHost_mac.cpp uses API deprecated in iOS 7, crbug.com/408571
     cflags = [ "-Wno-deprecated-declarations" ]
   }
+
+  if (is_clang) {
+    # Skia won't build with the enum bitfield warning in Clang, crbug.com/648462
+    cflags += [ "-Wno-signed-enum-bitfield" ]
+  }
 }
 
 # Separated out so it can be compiled with different flags for SSE.