Some small followup cleanups resulting from https://codereview.chromium.org/77363002/

Review URL: https://codereview.chromium.org/76953003

git-svn-id: svn://svn.chromium.org/blink/trunk@163477 bbb929c8-8fbe-4397-9dbb-9b2b20218538
Remove unnecessary static_cast<>s in DataTransferItemFielSystem.cpp

BUG=

Review URL: https://codereview.chromium.org/102683003

git-svn-id: svn://svn.chromium.org/blink/trunk@163478 bbb929c8-8fbe-4397-9dbb-9b2b20218538