Exclude third-party code in blink from yapf

The whole third_party/blink is covered by third_party/blink/.style.yapf,
but it also has some third-party dependencies in it. Unfortunately,
adding ignore patterns to the top-level .yapfignore seems to be the
easiest way to skip these directories.

Bug: 846432
Change-Id: I02428d30697745fa1424be4faa147a687554bc5c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2052547
Reviewed-by: Andrew Grieve <agrieve@chromium.org>
Commit-Queue: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740832}

.yapfignore

@@ -1,2 +1,6 @@
 # Generated protobuf code.
 *_pb2.py
+
+# Third-party code.
+third_party/blink/tools/blinkpy/third_party/*
+third_party/blink/web_tests/external/wpt/*