Move chromium/PasteboardChromium.cpp to Pasteboard.cpp.

There are no other implementations.

BUG=276565

Review URL: https://chromiumcodereview.appspot.com/23283015

git-svn-id: svn://svn.chromium.org/blink/trunk@156475 bbb929c8-8fbe-4397-9dbb-9b2b20218538