roll GN

The GN we sync is kind of old, 456551.  This rolls up to 514519.

I don't foresee any interesting effect from this roll.  I just happened
to notice that my Skia GN binary can't work with Chromium's configs on
Mac anymore.

Change-Id: I592b57747a1a43fbffc11d09d9f49281bad70418
Reviewed-on: https://skia-review.googlesource.com/76060
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/DEPS b/DEPS
index 78f46e5..c77d207 100644
--- a/DEPS
+++ b/DEPS
@@ -1,7 +1,7 @@
 use_relative_paths = True
 
 deps = {
-  "buildtools"                          : "https://chromium.googlesource.com/chromium/buildtools.git@e6b510a9daf822bbe9f922c200c58150803d2fd8",
+  "buildtools"                          : "https://chromium.googlesource.com/chromium/buildtools.git@505de88083136eefd056e5ee4ca0f01fe9b33de8",
   "common"                              : "https://skia.googlesource.com/common.git@9737551d7a52c3db3262db5856e6bcd62c462b92",
   "third_party/externals/angle2"        : "https://chromium.googlesource.com/angle/angle.git@ebee5b3b7de574c434cd48679a6ab85c6b10b077",
   "third_party/externals/dng_sdk"       : "https://android.googlesource.com/platform/external/dng_sdk.git@96443b262250c390b0caefbf3eed8463ba35ecae",