Add mojo struct traits for GURL so that it can be sent over mojoms.

url/mojo/url_gurl_struct_traits.h

Index: url/mojo/url_gurl_struct_traits.h
diff --git a/url/mojo/url_gurl_struct_traits.h b/url/mojo/url_gurl_struct_traits.h
new file mode 100644
index 0000000000000000000000000000000000000000..88b1dae4d2d9018837cc8ffe552610df8b080714
--- /dev/null
+++ b/url/mojo/url_gurl_struct_traits.h
@@ -0,0 +1,39 @@
+// Copyright 2016 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "base/strings/string_piece.h"
+#include "url/gurl.h"
+#include "url/mojo/url.mojom.h"
+
+namespace mojo {
+
+// copied from content/public/common/content_constants.cc: make that file use
+// this definition.
+const size_t kMaxUrlChars = 2 * 1024 * 1024;
+
+template <>
+struct StructTraits<url::mojom::Url, GURL> {
+  static base::StringPiece url(const GURL& r) {

[dcheng, 2016/03/03 21:37:36]

I don't think I understand how the magic name for this is determined, but that
shouldn't block this review.

[jam, 2016/03/03 21:49:47]

agree we need better documentation

+    if (r.possibly_invalid_spec().length() > kMaxUrlChars || !r.is_valid()) {
+      return base::StringPiece();
+    }
+
+    return base::StringPiece(r.possibly_invalid_spec().c_str(),
+                             r.possibly_invalid_spec().length());
+  }
+  static bool Read(url::mojom::Url::Reader r, GURL* out) {
+    if (r.url().length() > kMaxUrlChars) {
+      *out = GURL();
+      return false;
+    }
+    *out = GURL(r.url());
+    if (!r.url().empty() && !out->is_valid()) {
+      *out = GURL();
+      return false;
+    }
+    return true;
+  }
+};
+
+}