Cleanup namespace usage in Source/core/{dom/* and fileapi/*}

Cleanup namespace usage in Source/core/{dom/* and fileapi/*}

- Remove unnecessary blink:: prefixes
- Make blank lines consistent

BUG=None

Review URL: https://codereview.chromium.org/460393003

git-svn-id: svn://svn.chromium.org/blink/trunk@180182 bbb929c8-8fbe-4397-9dbb-9b2b20218538