Split SelLdrLauncher into SelLdrLauncher{Standalone,Chrome} classes

Split SelLdrLauncher into SelLdrLauncher{Standalone,Chrome} classes

This will make it easier to move sel_ldr_launcher_chrome.cc into the
Chromium repo.

This removes the need for the use of "#ifdef NACL_STANDALONE".

This removes these unused bits:

 * The SelLdrLauncher constructor that takes a SelLdrLocator.
 * The Start() method that takes a socket_count argument.

BUG=http://code.google.com/p/nativeclient/issues/detail?id=2750
TEST=trybots + build Chromium

Committed: https://src.chromium.org/viewvc/native_client?view=rev&revision=8444

[Mark Seaborn, 2012-04-27 00:51:36]

This depends on http://codereview.chromium.org/10192003/ being DEPS'd into NaCl.

[sehr (please use chromium), 2012-04-27 20:33:09]

LGTM and nicely done!