Roll buildtools 282996b8c3..1be57dc4c2

  In order to roll GN f3a5b8c1..a37803c9 (r520777:r523254) and pick up
  the following changes:

  a37803c9 Don't print warning for unused default args
  06ebd496 GN: don't deadlock when encountering import loops
  e48b26a4 Remove base::Value::IsType
  6713d85d Generate relative paths for build.ninja and build.ninja.d

TBR=dpranke@chromium.org

Change-Id: I1cb88049988dd4c3ccfd858909a31830cd9acb8c
Reviewed-on: https://chromium-review.googlesource.com/821210Reviewed-by: Scott Graham <scottmg@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Scott Graham <scottmg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#523519}

DEPS

@@ -90,7 +90,7 @@ 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': '282996b8c3cf7f05bbb5209432e9fe8162c6e7ac',
+  'buildtools_revision': '1be57dc4c2c68fa564ee258a3af0cf8003007edf',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling SwiftShader
   # and whatever else without interference from each other.