Roll GN

This picks up https://chromium.googlesource.com/chromium/src/tools/gn/+/efc976e5029419fde92111ce52f913c61fb9553c, which means we don't need to create a link_pool any more.  The default behavior is unlimited linking, what we were asking for explicitly.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2288253002

Review-Url: https://codereview.chromium.org/2288253002
diff --git a/DEPS b/DEPS
index 4f1afc1..6d00116 100644
--- a/DEPS
+++ b/DEPS
@@ -3,7 +3,7 @@
 # Dependencies on outside packages.
 #
 deps = {
-  "buildtools":  "https://chromium.googlesource.com/chromium/buildtools.git@33a32b8aa2b7274d246fcf85ce8f762cf4291418",
+  "buildtools":  "https://chromium.googlesource.com/chromium/buildtools.git@adb8bf4e8fc92aa1717bf151b862d58e6f27c4f2",
 
   "common": "https://skia.googlesource.com/common.git@c282fe0b6e392b14f88d647cbd86e1a3ef5498e0",