Roll buildtools 1a6b666652..8dd3c8e39a

  In order to roll GN a8e5d2e3..c0f67c55 (r396969:r398260) and pick up
  the following changes:

  40451c50 Add support for user defined "pool" to GN.
  0e85587e Fix doc on "Multiple outputs" for runtime_deps.
  57979207 [Mac/iOS/GN] Document the new rules for create_bundle and runtime_deps.
  4feb71fc [GN] Adding missing source type for extension .appex.
  3dbb1f62 Migrate WaitableEvent to enum-based constructor in tools/

TBR=dpranke@chromium.org
CQ_EXTRA_TRYBOTS=tryserver.chromium.mac:mac_chromium_gn_dbg;tryserver.chromium.win:win8_chromium_gn_dbg

Review-Url: https://codereview.chromium.org/2045913002
Cr-Commit-Position: refs/heads/master@{#398269}

DEPS

@@ -55,7 +55,7 @@ vars = {
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling build tools
   # and whatever else without interference from each other.
-  'buildtools_revision': '1a6b6666528b3428abf1cbee7a3b82a30c0355ee',
+  'buildtools_revision': '8dd3c8e39a48743c4eb790d9a8abe9676b617046',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling PDFium
   # and whatever else without interference from each other.