Cleanup namespace usage in Source/web/*.h

- Merge multiple |namespace blink| blocks
- Remove unnecessary blink:: prefix
- Other minor style fixes.

BUG=

Review URL: https://codereview.chromium.org/474183002

git-svn-id: svn://svn.chromium.org/blink/trunk@180427 bbb929c8-8fbe-4397-9dbb-9b2b20218538