checklicenses: Add a suppression for third_party/pdfium.

Will try to resolve soon.

BUG=374943
NOTRY=true
TBR=jam@chromium.org

Review URL: https://codereview.chromium.org/294773002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@271495 0039d316-1c4b-4281-b951-d872f2087c98

tools/checklicenses/checklicenses.py

@@ -260,6 +260,9 @@ PATH_SPECIFIC_WHITELISTED_LICENSES = {
     'third_party/ocmock/OCMock': [  # http://crbug.com/98454
         'UNKNOWN',
     ],
+    'third_party/pdfium': [  # http://crbug.com/374943
+        'UNKNOWN',
+    ],
     'third_party/ply/__init__.py': [
         'UNKNOWN',
     ],