Disable Clang extra semicolon warning.

Usually this warning is disabled by default. But when turned on Clang
complains about extra semicolons in Glslang headers. Turn this off for
now. See http://crbug.com/926235

BUILD.gn

@@ -141,6 +141,7 @@ source_set("glslang_sources") {
 
   if (is_clang) {
     cflags_cc = [
+      "-Wno-extra-semi",
       "-Wno-ignored-qualifiers",
       "-Wno-implicit-fallthrough",
       "-Wno-inconsistent-missing-override",