Add COMPONENT field to third_party/WebKit/OWNERS

This CL has no behavior changes.

NOTRY=true

Review-Url: https://codereview.chromium.org/2538333003
Cr-Commit-Position: refs/heads/master@{#435570}

third_party/WebKit/OWNERS

@@ -9,3 +9,5 @@ per-file .gitignore=*
 per-file DEPS=*
 per-file PRESUBMIT*.py=dpranke@chromium.org
 per-file PRESUBMIT*.py=thakis@chromium.org
+
+# COMPONENT: Blink