Fix browser_tests failure.

chrome/browser/chromeos/input_method/xkeyboard.h

Index: chrome/browser/chromeos/input_method/xkeyboard.h
diff --git a/chrome/browser/chromeos/input_method/xkeyboard.h b/chrome/browser/chromeos/input_method/xkeyboard.h
index f72325463e6af969521b79756df6073008b4958a..8445b2c94b2a16d6addaab2e313617cc66b87307 100644
--- a/chrome/browser/chromeos/input_method/xkeyboard.h
+++ b/chrome/browser/chromeos/input_method/xkeyboard.h
@@ -111,7 +111,8 @@ class XKeyboard {
       const std::string& layout_name,
       const ModifierMap& modifire_map) = 0;
 
-  // Returns a mask (e.g. 1U<<4) for Num Lock. On error, returns 0.
+  // Returns a mask (e.g. 1U<<4) for Num Lock. On error, returns 0. Do not call
+  // the function from non-UI threads.
   // TODO(yusukes): Move this and webdriver::GetXModifierMask() functions in
   // chrome/test/webdriver/keycode_text_conversion_x.cc to ui/base/x/x11_util.
   // The two functions are almost the same.