Cleanup namespace usage in Source/web/[A-V]*.h

This patch removes unnecessary blink:: prefixes.
Also it merges *namspace blink{}* blocks.

BUG=

Review URL: https://codereview.chromium.org/462613002

git-svn-id: svn://svn.chromium.org/blink/trunk@179959 bbb929c8-8fbe-4397-9dbb-9b2b20218538