Devirtualize DataTransferItemList and DataTransferItem

There's no need to separate the code into non-Chromium and Chromium bits
anymore. The embedder provides the abstraction around the platform APIs
for accessing the copy and paste/drag and drop pasteboards, and Blink
can use that abstraction directly.

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/24644006

git-svn-id: svn://svn.chromium.org/blink/trunk@158390 bbb929c8-8fbe-4397-9dbb-9b2b20218538