Move content_settings_custom_extension_provider.* to c/b/extensions/api/content_settings/

This CL moves content_settings_custom_extension_provider.* to
chrome/browser/extensions/api/content_settings/. The detailed discussion
can be seen here: https://code.google.com/p/chromium/issues/detail?id=393202

BUG=384868

Review URL: https://codereview.chromium.org/748723002

Cr-Commit-Position: refs/heads/master@{#305408}
5 files changed