Refactor to support PNaCl
Refactor to support PNaCl

This CL refactors the build script to support PNaCl.  It also
contains a fix for the missing (shell=True) which causes Windows to fail.

Windows uses scons.bat which does not get interpreted
unless the command is run through the shell.
Review URL: https://chromiumcodereview.appspot.com/9388021

git-svn-id: de016e52bd170d2d4f2344f9bf92d50478b649e0@121775 0039d316-1c4b-4281-b951-d872f2087c98

gavinp/chromium,gavinp/chromium,gavinp/chromium,yitian134/chromium,ropik/chromium,adobe/chromium,adobe/chromium,adobe/chromium,ropik/chromium,yitian134/chromium,gavinp/chromium,yitian134/chromium,adobe/chromium,ropik/chromium,adobe/chromium,gavinp/chromium,yitian134/chromium,gavinp/chromium,ropik/chromium,yitian134/chromium,adobe/chromium,ropik/chromium,yitian134/chromium,adobe/chromium,gavinp/chromium,adobe/chromium,yitian134/chromium,adobe/chromium,ropik/chromium,yitian134/chromium,gavinp/chromium,gavinp/chromium,gavinp/chromium,ropik/chromium,yitian134/chromium,ropik/chromium,yitian134/chromium,adobe/chromium,adobe/chromium,ropik/chromium