Initial switch to Chromium-style CHECK_* and DCHECK_* macros.

src/ast.h

Index: src/ast.h
diff --git a/src/ast.h b/src/ast.h
index 02f7df64b0d62bbaa9c1eb717ce1db6f8d4939b5..e7175ba5879d001eeb5bfa08d1acb238dadfa015 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -1651,7 +1651,7 @@ class VariableProxy FINAL : public Expression {
   }
   void set_var(Variable* v) {
     DCHECK(!is_resolved());
-    DCHECK_NOT_NULL(v);
+    DCHECK(v);
     var_ = v;
   }