b=587304; fix some string usage in WebGL shader validation; r=jeff

content/canvas/src/WebGLContextGL.cpp

@@ -2860,7 +2860,7 @@ WebGLContext::CompileShader(nsIWebGLShader *sobj)
 
         compiler = ShConstructCompiler(lang, EShSpecWebGL, &resources);
 
-        nsDependentCString src(shader->Source());
+        nsPromiseFlatCString src(shader->Source());
         const char *s = src.get();
 
         if (!ShCompile(compiler, &s, 1, EShOptSimple, debugFlags)) {