Move DOM callback logic out of ChromiumDataObject::getAsString.

This can just live in the caller, DataTransferItem.

Rename ChromiumDataObject::internalGetAsString to getAsString.

BUG=328079

Review URL: https://codereview.chromium.org/100863007

git-svn-id: svn://svn.chromium.org/blink/trunk@164103 bbb929c8-8fbe-4397-9dbb-9b2b20218538