Add a policy and pref to control whether deleting browsing and download history is allowed.

chrome/common/pref_names.cc

Index: chrome/common/pref_names.cc
===================================================================
--- chrome/common/pref_names.cc	(revision 181669)
+++ chrome/common/pref_names.cc	(working copy)
@@ -914,6 +914,10 @@
 // Boolean controlling whether history saving is disabled.
 const char kSavingBrowserHistoryDisabled[] = "history.saving_disabled";
 
+// Boolean controlling whether deleting browsing and download history is
+// permitted.
+const char kDeletingBrowserHistoryEnabled[] = "history.deleting_enabled";
+
 // Boolean controlling whether SafeSearch is mandatory for Google Web Searches.
 const char kForceSafeSearch[] = "settings.force_safesearch";