Update build tools to newer gn.

Pasted from Chromium DEPS on 3/17/17.

R=reed@google.com

Change-Id: Ie1b93fadbad9e24b4a547f237df602a7257138f7
Reviewed-on: https://skia-review.googlesource.com/9831
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Reed <reed@google.com>
diff --git a/DEPS b/DEPS
index 7b5dc1d..2dc262d 100644
--- a/DEPS
+++ b/DEPS
@@ -1,7 +1,7 @@
 use_relative_paths = True
 
 deps = {
-  "buildtools"                          : "https://chromium.googlesource.com/chromium/buildtools.git@55ad626b08ef971fd82a62b7abb325359542952b",
+  "buildtools"                          : "https://chromium.googlesource.com/chromium/buildtools.git@e6b510a9daf822bbe9f922c200c58150803d2fd8",
   "common"                              : "https://skia.googlesource.com/common.git@9737551d7a52c3db3262db5856e6bcd62c462b92",
   "third_party/externals/angle2"        : "https://chromium.googlesource.com/angle/angle.git@57f17473791703ac82add77c3d77d13d8e2dfbc4",
   "third_party/externals/dng_sdk"       : "https://android.googlesource.com/platform/external/dng_sdk.git@96443b262250c390b0caefbf3eed8463ba35ecae",