Adds a pref and a policy to decide if PDFs should always be opened externally.

chrome/common/pref_names.cc

Index: chrome/common/pref_names.cc
diff --git a/chrome/common/pref_names.cc b/chrome/common/pref_names.cc
index bde96d1fceea656091082eb613fb6bc93d7758ef..97a22ec799ffc13bcc775b6ff1783f47ffe41d1e 100644
--- a/chrome/common/pref_names.cc
+++ b/chrome/common/pref_names.cc
@@ -980,6 +980,10 @@ const char kPluginsDisabledPluginsExceptions[] =
 // List pref containing names of plugins that are enabled by policy.
 const char kPluginsEnabledPlugins[] = "plugins.plugins_enabled";
 
+// Whether Chrome should use its internal PDF viewer or not.
+const char kPluginsAlwaysOpenPdfExternally[] =
+    "plugins.always_open_pdf_externally";
+
 #if defined(ENABLE_PLUGINS)
 // Whether about:plugins is shown in the details mode or not.
 const char kPluginsShowDetails[] = "plugins.show_details";