Bug 1526581 [wpt PR 15249] - CODEOWNERS cleanup, a=testonly

Automatic update from web-platform-tests
Change CODEOWNERS to only cover the subtree in /css/tools

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Add /tools/third_party to CODEOWNERS

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wpt-commits: c70ddef5327359fe64d4b63260e5cd64c5b26db1, 5a406c55ff70128e0661209d1d169c4104d14eed
wpt-pr: 15249

testing/web-platform/tests/CODEOWNERS

@@ -1,2 +1,5 @@
-# Prevent accidentially touching CSS submodules
-/css/tools/ @plinss @kojiishi @jgraham @gsnedders
+# Prevent accidentially touching CSS subtree
+/css/tools/apiclient/ @plinss @jgraham @gsnedders
+
+# Prevent accidentally touching tools/third_party
+/tools/third_party/ @jgraham @gsnedders