Add chrome/installer/mac/third_party/xz, and update DEPS to add r50504 on Mac OS X

DEPS

 vars = {
   "webkit_trunk":
   "http://svn.webkit.org/repository/webkit/trunk",
   "webkit_revision": "61547",
   "ffmpeg_revision": "49485",
   "skia_revision": "582",
   "chromium_git": "http://src.chromium.org/git",
   "swig_revision": "40423",
   "nacl_revision": "2555",
   "libvpx_revision": "47941",
@@ skipping 212 matching lines @@
     # TODO(ajwong): Move this into the OS-independent block.
     "src/third_party/yasm/source/patched-yasm":
       "/trunk/deps/third_party/yasm/patched-yasm@29937",
 
     "src/third_party/swig/mac":
       "/trunk/deps/third_party/swig/mac@" + Var("swig_revision"),
 
     # NSS, for SSLClientSocketNSS.
     "src/third_party/nss":
       "/trunk/deps/third_party/nss@48440",
+
+    "src/chrome/installer/mac/third_party/xz/xz":
+      "/trunk/deps/third_party/xz@50504",
   },
   "unix": {
     # Linux, really.
     "src/chrome/tools/test/reference_build/chrome_linux":
       "/trunk/deps/reference_builds/chrome_linux@41515",
 
     "src/third_party/xdg-utils":
       "/trunk/deps/third_party/xdg-utils@29103",
 
     # TODO(ajwong): Move this into the OS-independent block.
@@ skipping 43 matching lines @@
   },
   {
     # Workaround XCode dependecies problem.
     # TODO(bradnelson): remove this when fixed.
     "pattern": ".",
     "action": ["python",
                "src/build/mac/clobber_generated_headers.py"],
   },
 ]