Add FakeClipboard implementation for unit tests.

ui/base/BUILD.gn

Index: ui/base/BUILD.gn
diff --git a/ui/base/BUILD.gn b/ui/base/BUILD.gn
index 07dbec017a16a10c223862a92b1873d15b4fb352..6374ce46f2174f8c1ca3b31585a21e5e6e9d99c9 100644
--- a/ui/base/BUILD.gn
+++ b/ui/base/BUILD.gn
@@ -656,6 +656,8 @@ component("base") {
 source_set("test_support") {
   testonly = true
   sources = [
+    "test/test_clipboard.cc",
+    "test/test_clipboard.h",
     "test/ui_controls.h",
     "test/ui_controls_aura.cc",
     "test/ui_controls_internal_win.cc",
@@ -728,6 +730,7 @@ test("ui_unittests") {
     "resource/data_pack_literal.cc",
     "resource/data_pack_unittest.cc",
     "resource/resource_bundle_unittest.cc",
+    "test/test_clipboard_unittest.cc",
     "test/run_all_unittests.cc",
   ]