ninja: Don't assume that depot_tools is in PATH.
ninja: Don't assume that depot_tools is in PATH.

TEST=`~/src/depot_tools/ninja --version` works when depot_tools isn't in PATH

Review URL: https://chromiumcodereview.appspot.com/11366044

git-svn-id: bd64dd6fa6f3f0ed0c0666d1018379882b742947@165510 4ff67af0-8c30-449e-8e8b-ad334ec8d88c

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