Add runtime setting to force passive event listeners.

content/public/common/content_switches.cc

 // Copyright (c) 2012 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "build/build_config.h"
 #include "content/public/common/content_switches.h"
 
 namespace switches {
 
 // The number of MSAA samples for canvas2D. Requires MSAA support by GPU to
@@ skipping 657 matching lines @@
 // Set the value to '1' to enable the behavior where pages slide in and out in
 // response to the horizontal overscroll gesture and a screenshot of the target
 // page is shown.
 // Set the value to '2' to enable the simplified overscroll UI where a
 // navigation arrow slides in from the side of the screen in response to the
 // horizontal overscroll gesture.
 // Defaults to '1'.
 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";
 
 // "Command-line" arguments for the PPAPI Flash; used for debugging options.
 const char kPpapiFlashArgs[]                = "ppapi-flash-args";
 
 // Runs PPAPI (Pepper) plugins in-process.
 const char kPpapiInProcess[]                = "ppapi-in-process";
 
 // Specifies a command that should be used to launch the ppapi plugin process.
@@ skipping 343 matching lines @@
 const char kMemoryPressureThresholdsMb[] = "memory-pressure-thresholds-mb";
 
 // Enables the exporting of the tracing events to ETW. This is only supported on
 // Windows Vista and later.
 const char kTraceExportEventsToETW[] = "trace-export-events-to-etw";
 #endif
 
 // Don't dump stuff here, follow the same order as the header.
 
 }  // namespace switches