Roll buildtools 277db9302c..818123dac3

  In order to roll GN 8b4796b5..e8853384 (r359874:r360325) and pick up
  the following changes:

    354261e7 Sort "deps" and "public_deps" when running the "gn format" command.
    5b67bfea fix gn bootstrap once again
    45e4d295 tools/gn: mark first TestTarget parameter as const

TBR=dpranke@chromium.org
CQ_EXTRA_TRYBOTS=tryserver.chromium.mac:mac_chromium_gn_dbg;tryserver.chromium.win:win8_chromium_gn_dbg,win_chromium_gn_x64_rel

Review URL: https://codereview.chromium.org/1460633003

Cr-Commit-Position: refs/heads/master@{#360504}

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': '277db9302c7ff55beee2a9daf380675dc03bea83',
+  'buildtools_revision': '818123dac34899ec230840936fc15b8b2b5556f9',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling PDFium
   # and whatever else without interference from each other.