ui/base: Move message_box_win.{h,cc} to ui/base/win.

base/base_switches.cc

Index: base/base_switches.cc
diff --git a/base/base_switches.cc b/base/base_switches.cc
index 78f55a707ea39be01740c0d04e04bee3216f5b07..65aa7ed839c281b526ee2201f3387d462362615a 100644
--- a/base/base_switches.cc
+++ b/base/base_switches.cc
@@ -24,10 +24,6 @@ const char kFullMemoryCrashReport[]         = "full-memory-crash-report";
 // Suppresses all error dialogs when present.
 const char kNoErrorDialogs[]                = "noerrdialogs";
 
-// Disable ui::MessageBox.  This is useful when running as part of scripts that
-// do not have a user interface.
-const char kNoMessageBox[]                  = "no-message-box";
-
 // When running certain tests that spawn child processes, this switch indicates
 // to the test framework that the current process is a child process.
 const char kTestChildProcess[]              = "test-child-process";