Add LayoutThemeFuchsia theme stub to Blink.

Add LayoutThemeFuchsia theme stub to Blink.
The theme is necessary because no LayoutTheme* files are currently
built when is_fuchsia=true.

Bug: 750946
Change-Id: Iab9cdf66856164309882239e643c2006f200d669
Reviewed-on: https://chromium-review.googlesource.com/599248Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Scott Graham <scottmg@chromium.org>
Commit-Queue: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#491626}

third_party/WebKit/Source/core/layout/BUILD.gn

@@ -491,6 +491,10 @@ blink_core_sources("layout") {
     ]
   }
 
+  if (is_fuchsia) {
+    sources += [ "LayoutThemeFuchsia.cpp" ]
+  }
+
   configs += [
     # TODO(jschuh): crbug.com/167187 fix size_t to int truncations.
     "//build/config/compiler:no_size_t_to_int_warning",

third_party/WebKit/Source/core/layout/LayoutThemeFuchsia.cpp

+// Copyright 2017 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 "core/layout/LayoutThemeDefault.h"
+
+namespace blink {
+namespace {
+
+// Fuchsia is headless-only for now, so no native themes are applied.
+// TODO(fuchsia): Implement this when we enable the UI. (crbug.com/750946)
+class LayoutThemeFuchsia : public LayoutThemeDefault {
+ public:
+  static RefPtr<LayoutTheme> Create() {
+    return AdoptRef(new LayoutThemeFuchsia());
+  }
+};
+
+}  // namespace
+
+LayoutTheme& LayoutTheme::NativeTheme() {
+  DEFINE_STATIC_REF(LayoutTheme, layout_theme, (LayoutThemeFuchsia::Create()));
+  return *layout_theme;
+}
+
+}  // namespace blink