Roll buildtools 64e38f0ceb..55ad626b08

  In order to roll GN 78660e8..8897c83 (r436326:r436733) and pick up
  the following changes:

  4c53f32 gn: Fix integer subtraction.
  d177088 gn: Fix stale references in Parser::AssignComments().
  e591a5f gn: Add spelling suggestions for args.gn and --args.
  b0d8fb4 gn: Remove redundant calls to Scheduler::Increment/DecrementWorkCount().
  5476ec6 gn: Remove Scheduler::pool().

TBR=dpranke@chromium.org

Review-Url: https://codereview.chromium.org/2553233002
Cr-Commit-Position: refs/heads/master@{#436778}

DEPS

@@ -56,7 +56,7 @@ 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': '64e38f0cebdde27aa0cfb405f330063582f9ac76',
+  'buildtools_revision': '55ad626b08ef971fd82a62b7abb325359542952b',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling SwiftShader
   # and whatever else without interference from each other.