Add API to specify command-line switches

Normally, chromium gets the list of command-line switches directly from the
Win32 command line.  This commit adds an API that lets applications add
command-line switches programatically.
Reimplement disablePluginDiscovery in terms of appendCommandLineSwitch
Remove 'sandboxDisabled' flag in favor of appendCommandLineSwitch
Pass the right userData when canceling resource loader
Add support for rubberbanding in blpwtk2

This requires the 'feature/rubberband' branch to be merged, otherwise it is a
no-op.