[Linux] add MockSignalHandler for testing GObject signals

This is a simple helper class that allows settings GMock expectations
on GObject signals:

```c
flutter::testing::MockSignalHandler foo_changed(foo_object, "changed");
EXPECT_SIGNAL(foo_changed).Times(1);

foo_set_something(foo_object, ...);
```

Author: jpnurmi

shell/platform/linux/BUILD.gn

@@ -208,6 +208,7 @@ executable("flutter_linux_unittests") {
     "testing/mock_epoxy.cc",
     "testing/mock_plugin_registrar.cc",
     "testing/mock_renderer.cc",
+    "testing/mock_signal_handler.cc",
     "testing/mock_text_input_plugin.cc",
     "testing/mock_texture_registrar.cc",
   ]

shell/platform/linux/testing/mock_signal_handler.cc

@@ -0,0 +1,30 @@
+// Copyright 2013 The Flutter 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 "flutter/shell/platform/linux/testing/mock_signal_handler.h"
+
+#include <glib-object.h>
+
+namespace flutter {
+namespace testing {
+
+MockSignalHandler::MockSignalHandler(gpointer instance, const gchar* name) {
+  id_ = g_signal_connect_data(instance, name, G_CALLBACK(OnSignal), this,
+                              nullptr, G_CONNECT_SWAPPED);
+  g_object_add_weak_pointer(G_OBJECT(instance), &instance_);
+}
+
+MockSignalHandler::~MockSignalHandler() {
+  if (instance_) {
+    g_signal_handler_disconnect(instance_, id_);
+    g_object_remove_weak_pointer(G_OBJECT(instance_), &instance_);
+  }
+}
+
+void MockSignalHandler::OnSignal(MockSignalHandler* mock) {
+  mock->Handler();
+}
+
+}  // namespace testing
+}  // namespace flutter

shell/platform/linux/testing/mock_signal_handler.h

@@ -0,0 +1,42 @@
+// Copyright 2013 The Flutter 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 FLUTTER_SHELL_PLATFORM_LINUX_MOCK_SIGNAL_HANDLER_H_
+#define FLUTTER_SHELL_PLATFORM_LINUX_MOCK_SIGNAL_HANDLER_H_
+
+#include <glib.h>
+
+#include "gmock/gmock.h"
+
+// Sets an expectation on the mock signal handler.
+//
+// For example:
+//
+// MockSignalHandler foo_changed(foo, "changed");
+// EXPECT_SIGNAL(foo_changed).Times(3);
+//
+#define EXPECT_SIGNAL(mock) EXPECT_CALL(mock, Handler())
+
+namespace flutter {
+namespace testing {
+
+// Allows setting expectations on signals. See EXPECT_SIGNAL() above.
+class MockSignalHandler {
+ public:
+  MockSignalHandler(gpointer instance, const gchar* name);
+  ~MockSignalHandler();
+
+  MOCK_METHOD0(Handler, void());
+
+ private:
+  static void OnSignal(MockSignalHandler* mock);
+
+  gulong id_ = 0;
+  gpointer instance_ = nullptr;
+};
+
+}  // namespace testing
+}  // namespace flutter
+
+#endif  // FLUTTER_SHELL_PLATFORM_LINUX_MOCK_SIGNAL_HANDLER_H_