Fully specify the path to the port command.

For some reason, a few people's macs don't have port on sudo's path by default.

BUG=
R=kulakowski@chromium.org

Review URL: https://codereview.chromium.org/1212373008.
diff --git a/build/install-build-deps-mac.sh b/build/install-build-deps-mac.sh
index 961786b..625ddf4 100755
--- a/build/install-build-deps-mac.sh
+++ b/build/install-build-deps-mac.sh
@@ -44,7 +44,7 @@
 if [ ! -z $(which brew) ]; then
   brew install ant
 elif [ ! -z $(which port) ]; then
-  sudo port install apache-ant
+  sudo `which port` install apache-ant
 else
   INSTRUCTIONS="${INSTRUCTIONS}\n** Install homebrew (brew.sh) or macports (macports.org) and re-run this script."
 fi
@@ -61,4 +61,4 @@
   echo
   echo "**** Follow these final instructions to get going."
   echo $INSTRUCTIONS
-fi
\ No newline at end of file
+fi