Cleanup namespace usage in public/platform/{Platform,Web[A-M]*}.h.

- Merge multiple |namespace blink {}| blocks
- Remove unnecessary blink:: prefixes
- Make blank lines consistent

BUG=

Review URL: https://codereview.chromium.org/457353002

git-svn-id: svn://svn.chromium.org/blink/trunk@179942 bbb929c8-8fbe-4397-9dbb-9b2b20218538