Move clipboard helpers from core/platform/chromium into platform.

To make the move easier (and clean up the code), I've removed
currentPasteboardBuffer() in favor of simply using Pasteboard::buffer().
The old code was silly anyway since it resulted in a conversion from
an enum value to a bool and then a bool back to an enum value.

BUG=297477
R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/25560005

git-svn-id: svn://svn.chromium.org/blink/trunk@158767 bbb929c8-8fbe-4397-9dbb-9b2b20218538