Move all GYP->GN argument conversion to Python.

This moves build variable remapping from C++ to the Python wrapper. This will be much more scaleable since we can add stuff to the script atomically without rolling out new binaries.

R=scottmg@chromium.org, scottmg

Review URL: https://codereview.chromium.org/111783003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240732 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240732
Revert of https://chromiumcodereview.appspot.com/107163006/
Reason for revert: Reverting the revert.  Canary failure not due to this change.

TBR=tonyg@chromium.org,dtu@chromium.org
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/109653007

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@240733 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 240733