Process SelectionRequestor::PerformBlockingConvertSelection() requests one at a time

ui/ui_unittests.gyp

Index: ui/ui_unittests.gyp
diff --git a/ui/ui_unittests.gyp b/ui/ui_unittests.gyp
index 7b64deae50632335eb0bf82dbf4dbe1d9e82d9be..28f49339d8c81774fe702f0c031495f07920789e 100644
--- a/ui/ui_unittests.gyp
+++ b/ui/ui_unittests.gyp
@@ -66,6 +66,7 @@
         'base/text/bytes_formatting_unittest.cc',
         'base/view_prop_unittest.cc',
         'base/webui/web_ui_util_unittest.cc',
+        'base/x/selection_requestor_unittest.cc',
         'gfx/canvas_unittest_mac.mm',
         'gfx/platform_font_mac_unittest.mm',
       ],
@@ -212,6 +213,7 @@
           ],
           'sources!': [
             'base/dragdrop/os_exchange_data_provider_aurax11_unittest.cc',
+            'base/x/selection_requestor_unittest.cc',
           ],
         }],
       ],