Revert r144460 "Remove the linux-only CR_SOURCE_ROOT environment variable override of base::DIR_SOURCE_ROOT."

CR_SOURCE_ROOT is used in practice by linux user(s), plural still to be
determined, mostly for ChromeOS related development.

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BUG=
TEST=


Review URL: https://chromiumcodereview.appspot.com/10808048

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@148621 0039d316-1c4b-4281-b951-d872f2087c98
2 files changed