Roll GN

Mostly to get
    5d7f4ce [GN] Fix generate Xcode project for macOS to work with Xcode 8.

After this lands, gclient sync && python bin/fetch-gn to get the new GN.

Change-Id: I3e1a186996b1db69ef767922a5b9efff173b950c
Reviewed-on: https://skia-review.googlesource.com/5723
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
diff --git a/DEPS b/DEPS
index 5916f8d..802105d 100644
--- a/DEPS
+++ b/DEPS
@@ -3,7 +3,7 @@
 # Dependencies on outside packages.
 #
 deps = {
-  "buildtools":  "https://chromium.googlesource.com/chromium/buildtools.git@5fd66957f08bb752dca714a591c84587c9d70762",
+  "buildtools":  "https://chromium.googlesource.com/chromium/buildtools.git@55ad626b08ef971fd82a62b7abb325359542952b",
 
   "common": "https://skia.googlesource.com/common.git@9737551d7a52c3db3262db5856e6bcd62c462b92",