Plumb through buffer type for clipboard sequence number requests.

      Some minor cleanup to remove BUFFER_DRAG as well, which is not used anywhere.

BUG=73478
TEST=compiles

Review URL: http://codereview.chromium.org/8511042

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@109476 0039d316-1c4b-4281-b951-d872f2087c98