Ignore tools/json_schema_compiler from formatting by yapf.

git cl format formats the changed python code in
tools/json_schema_compiler using yapf since tools/ specifies a
style.yapf file. However this is proving to be a nuisance since yapf
doesn't seem to be doing a good job of formatting call chains and since
the directory is not formatted currently.

Exempt the directory from python formatting for now.

BUG=1113513, 1116155

Change-Id: I69b17ceeb0319fea8976c7b170ebdd33f6f12ce0
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2353418
Commit-Queue: Karan Bhatia <karandeepb@chromium.org>
Reviewed-by: Devlin <rdevlin.cronin@chromium.org>
Auto-Submit: Karan Bhatia <karandeepb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#797872}

.yapfignore

@@ -4,3 +4,6 @@
 # Third-party code.
 third_party/blink/tools/blinkpy/third_party/*
 third_party/blink/web_tests/external/wpt/*
+
+# TODO(crbug.com/1116155): Enable this for formatting by yapf.
+tools/json_schema_compiler/*