Allow unit tests to force serial execution in the test launcher.

ui/base/BUILD.gn

Index: ui/base/BUILD.gn
diff --git a/ui/base/BUILD.gn b/ui/base/BUILD.gn
index d11013d51bf6092c5046ac949ac3ca0961af232a..02ae3fb2abfcd0122a21a877a85b770ada559131 100644
--- a/ui/base/BUILD.gn
+++ b/ui/base/BUILD.gn
@@ -414,10 +414,9 @@ component("base") {
       "cursor/cursor_loader_win.h",
     ]
     deps += [ "//third_party/wtl" ]
-    cflags = [
-      "/wd4324",  # Structure was padded due to __declspec(align()), which is
-                  # uninteresting.
-    ]
+    cflags = [ "/wd4324" ]  # Structure was padded due to __declspec(align()), which is
+                            # uninteresting.
+
     ldflags = [
       "/DELAYLOAD:d2d1.dll",
       "/DELAYLOAD:d3d10_1.dll",
@@ -652,6 +651,7 @@ test("ui_base_unittests") {
     sources += [
       "accelerators/accelerator_manager_unittest.cc",
       "accelerators/menu_label_accelerator_util_linux_unittest.cc",
+      "clipboard/clipboard_unittest.cc",
       "clipboard/custom_data_helper_unittest.cc",
       "cocoa/base_view_unittest.mm",
       "cocoa/cocoa_base_utils_unittest.mm",