Fixed some code quality issues in SConstruct.
Fixed some code quality issues in SConstruct.

Fixed a few loops that iterated over dictionaries.
Replaced a few hackish statements with builtin Python functions.
Added two comments.

Review URL: http://codereview.chromium.org/4525007

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@3630 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client