checklicenses.py: Add "UNKNOWN" entry for third_party/blink

as well as third_party/WebKit because we'll move
third_party/WebKit/{Source,common,public} to third_party/blink soon.

Bug: 622551
Change-Id: Ib916a70651318dadc333416d8ca49927bceba1cb
Reviewed-on: https://chromium-review.googlesource.com/952946Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Kent Tamura <tkent@chromium.org>
Cr-Commit-Position: refs/heads/master@{#544995}

tools/checklicenses/checklicenses.py

@@ -168,6 +168,8 @@ PATH_SPECIFIC_WHITELISTED_LICENSES = {
         'UNKNOWN',
     ],
 
+    # TODO(tkent): Remove this entry after the move is completed.
+    # crbug.com/622551
     'third_party/WebKit': [
         'UNKNOWN',
     ],
@@ -182,6 +184,10 @@ PATH_SPECIFIC_WHITELISTED_LICENSES = {
         'UNKNOWN',
     ],
 
+    'third_party/blink': [
+        'UNKNOWN',
+    ],
+
     # https://crbug.com/google-breakpad/450
     'third_party/breakpad/breakpad': [
         'UNKNOWN',