bookmarks: bookmark_pasteboard_helper_mac.h misc clean ups.

- Move functions into global namespace.
- Rename the PasteboardType enum to BookmarkPasteboardType.
- Make BookmarkPasteboardType enum follow the content convention.
- Inline dragClipboardContainsBookmarks() directly into the call sites.

BUG=35966, 289619
TEST=None, no functional changes.
R=asvitkine@google.com, sky@chromium.org, asvitkine@chromium.org

Review URL: https://codereview.chromium.org/24916003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@225862 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 225862