Cleanup namespace usage in Source/core/platform/[A-Z]*.h

Cleanup namespace usage in Source/core/platform/[A-Z]*.h

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

BUG=None

Review URL: https://codereview.chromium.org/467193002

git-svn-id: svn://svn.chromium.org/blink/trunk@180218 bbb929c8-8fbe-4397-9dbb-9b2b20218538