Export the tablet mode checking function from base.

base/win/win_util.h

Index: base/win/win_util.h
diff --git a/base/win/win_util.h b/base/win/win_util.h
index b261ce7b5eae91dd12f826e124b552be92a18b44..947a12acc015b925f2987412559565924572229d 100644
--- a/base/win/win_util.h
+++ b/base/win/win_util.h
@@ -123,6 +123,10 @@ BASE_EXPORT bool ShouldCrashOnProcessDetach();
 // process is aborted.
 BASE_EXPORT void SetAbortBehaviorForCrashReporting();
 
+// Checks whether the supplied |hwnd| is in Windows 10 tablet mode. Will return
+// false on versions below 10.
+BASE_EXPORT bool IsWindows10TabletMode(HWND hwnd);
+
 // A tablet is a device that is touch enabled and also is being used
 // "like a tablet". This is used by the following:-
 // 1. Metrics:- To gain insight into how users use Chrome.