gyp_xwalk: Remove one TODO by doing the same as Chromium r252434.

This approach is much cleaner than filtering by GYP_GENERATORS and better
mirrors what's currently upstream in gyp_chromium.
Merge pull request #1747 from rakuco/gyp_xwalk-find-depot-tools

gyp_xwalk: Remove one TODO by doing the same as Chromium r252434.