Remove unused methods from WebClipboardImpl.

This also re-arranges the methods according to their definition in WebKit::WebClipboard.

BUG=

Review URL: https://codereview.chromium.org/25729002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@231036 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 231036