Revert of Roll buildtools 55638fe5c3..60f7f9a8b4 (patchset #1 id:1 of...

Revert of Roll buildtools 55638fe5c3..60f7f9a8b4 (patchset #1 id:1 of https://codereview.chromium.org/2166993002/ )

Reason for revert:
Speculative revert - this probably broke the official build - see crbug.com/630514 for more details. Will reland if the suspicion was wrong.

Original issue's description:
> Roll buildtools 55638fe5c3..60f7f9a8b4
>
>   In order to roll GN af92718f..1f417360 (r405547:r406705) and pick up
>   the following changes:
>
>   267b1de9 Make `gn refs` handle files in data directories properly.
>   5fc3fafd Add TaskPriority as a parameter to SequencedWorkerPool in preparation for TaskScheduler experiment.
>   aca08753 Make `gn refs` look at script files.
>   05316e06 Re-land r406064 "[GN] Add JSON project writer".
>   5a1b4b17 Revert of [GN] Add JSON project writer (patchset #11 id:200001 of https://codereview.chromium.org/2064533002/ )
>   d2edeb9a [GN] Add JSON project writer
>   f0544fc6 Remove calls to MessageLoop::current() in tools.
>   d93e5cf4 Document GN invoker and target name variables.
>
> TBR=dpranke@chromium.org
>
> Committed: https://crrev.com/d8a18dc3820a4d54d8fc9ad8667d242798f10d33
> Cr-Commit-Position: refs/heads/master@{#406727}

TBR=dpranke@chromium.org,brettw@chromium.org
# Not skipping CQ checks because original CL landed more than 1 days ago.

Review-Url: https://codereview.chromium.org/2171273002
Cr-Commit-Position: refs/heads/master@{#407125}

DEPS

@@ -52,7 +52,7 @@ 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': '60f7f9a8b421ebf9a46041dfa2ff11c0fe59c582',
+  'buildtools_revision': '55638fe5c31652a808f059027b99b08bafd15fb7',
   # Three lines of non-changing comments so that
   # the commit queue can handle CLs rolling PDFium
   # and whatever else without interference from each other.