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: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@165510 0039d316-1c4b-4281-b951-d872f2087c98

ninja

@@ -5,11 +5,12 @@
 # found in the LICENSE file.
 
 OS="$(uname -s)"
+THIS_DIR="$(dirname "${0}")"
 
 if [ "${OS}" = "Linux" ]; then
-  exec ninja-linux64 "$@"
+  exec "${THIS_DIR}/ninja-linux64" "$@"
 elif [ "${OS}" = "Darwin" ]; then
-  exec ninja-mac "$@"
+  exec "${THIS_DIR}/ninja-mac" "$@"
 elif [[ ${OS} == CYGWIN* ]]; then
   exec cmd.exe /c `cygpath -t windows $0`.exe "$@"
 elif [[ ${OS} == MINGW32* ]]; then