Add runtime setting to force passive event listeners.

content/public/common/content_switches.cc

Index: content/public/common/content_switches.cc
diff --git a/content/public/common/content_switches.cc b/content/public/common/content_switches.cc
index 6dbed9721353652b1ba5307b24814d4b5669f066..25602568d059d6ed814f58fe386a1a4a915a55f4 100644
--- a/content/public/common/content_switches.cc
+++ b/content/public/common/content_switches.cc
@@ -675,6 +675,12 @@ const char kOverridePluginPowerSaverForTesting[] =
 const char kOverscrollHistoryNavigation[] =
     "overscroll-history-navigation";
 
+// Default for passive listeners being added. Values are defined as:

[Charlie Reis, 2016/05/12 20:30:16]

Can you give some context for what passive listeners are in the comment?  It's
not clear from context.

[dtapuska, 2016/05/13 15:52:11]

Done.

+//  'documentonlytrue' to set the default be true only for document level nodes.
+//  'true' to set the default to be true on all nodes (when not specified).
+//  'forcealltrue' to force the value on all nodes.
+const char kPassiveListenersDefault[] = "passive-listeners-default";
+
 // Argument to the process type that indicates a PPAPI broker process type.
 const char kPpapiBrokerProcess[]            = "ppapi-broker";