Fixup syntax tree build.

The syntax tree stopped building with a recent addition which used `line`
instead of the new `Line`.

Change-Id: Ifc448c02825b35edf7b9a34e394ee23ccbf7bd62
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/139540
Reviewed-by: James Price <jrprice@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Kokoro: Ben Clayton <bclayton@google.com>
Commit-Queue: Dan Sinclair <dsinclair@chromium.org>
diff --git a/src/tint/writer/syntax_tree/generator_impl.cc b/src/tint/writer/syntax_tree/generator_impl.cc
index b6ba3ea..6ef2395 100644
--- a/src/tint/writer/syntax_tree/generator_impl.cc
+++ b/src/tint/writer/syntax_tree/generator_impl.cc
@@ -521,12 +521,12 @@
                 Line() << "]";
             },
             [&](const ast::IndexAttribute* index) {
-                line() << "IndexAttribute [";
+                Line() << "IndexAttribute [";
                 {
                     ScopedIndent idx(this);
                     EmitExpression(index->expr);
                 }
-                line() << "]";
+                Line() << "]";
             },
             [&](const ast::BuiltinAttribute* builtin) {
                 Line() << "BuiltinAttribute [";