Replace --ppapi-keep-alive-throttle command line switch with IPC parameter.

Part of a broad effort to reduce the number of command line switches.

BUG=350510

Review URL: https://codereview.chromium.org/191633002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@256436 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 256436