Get rid of custom code for clipboard.clearData()

Get rid of custom code for clipboard.clearData() by removing clearAllData() method
in the implementation and having clearData() behave as clearAllData() if the argument
is a null string.

There is no web-exposed behavior change.

R=haraken
TEST=fast/events/clipboard-clearData.html

Review URL: https://codereview.chromium.org/26023003

git-svn-id: svn://svn.chromium.org/blink/trunk@159141 bbb929c8-8fbe-4397-9dbb-9b2b20218538