Add UI_DEVTOOLS_EXPORT so tests can mock FrontendChannel

components/ui_devtools/devtools_export.h

Index: components/ui_devtools/devtools_export.h
diff --git a/components/ui_devtools/devtools_export.h b/components/ui_devtools/devtools_export.h
new file mode 100644
index 0000000000000000000000000000000000000000..1a94a2c50e8187d2b687c047497df31bd9937b63
--- /dev/null
+++ b/components/ui_devtools/devtools_export.h
@@ -0,0 +1,29 @@
+// 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.
+
+#ifndef COMPONENTS_UI_DEVTOOLS_EXPORT_H_
+#define COMPONENTS_UI_DEVTOOLS_EXPORT_H_
+
+#if defined(COMPONENT_BUILD)
+#if defined(WIN32)
+
+#if defined(UI_DEVTOOLS_IMPLEMENTATION)
+#define UI_DEVTOOLS_EXPORT __declspec(dllexport)
+#else
+#define UI_DEVTOOLS_EXPORT __declspec(dllimport)
+#endif  // defined(UI_DEVTOOLS_IMPLEMENTATION)
+
+#else  // defined(WIN32)
+#if defined(UI_DEVTOOLS_IMPLEMENTATION)
+#define UI_DEVTOOLS_EXPORT __attribute__((visibility("default")))
+#else
+#define UI_DEVTOOLS_EXPORT
+#endif
+#endif
+
+#else  // defined(COMPONENT_BUILD)
+#define UI_DEVTOOLS_EXPORT
+#endif
+
+#endif  // COMPONENTS_UI_DEVTOOLS_EXPORT_H_