Make Dependency.subtree() a standalone function for later use.
Make Dependency.subtree() a standalone function for later use.

No behavior change.

TEST=all test still pass
TBR=bradnelson

Review URL: http://codereview.chromium.org/3005026

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@53392 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

svn2github/chromium-depot-tools,svn2github/chromium-depot-tools,svn2github/chromium-depot-tools