Populate clipboard with only plain text if pasting as plain text.

The current behavior makes it impossible for paste handlers in webpages
to differentiate between normal and unstyled pastes. Instead, just don't
populate clipboardData with non-plaintext types when dispatching a paste
event triggered by an unstyled paste.

BUG=133033

Review URL: https://chromiumcodereview.appspot.com/24401002

git-svn-id: svn://svn.chromium.org/blink/trunk@158303 bbb929c8-8fbe-4397-9dbb-9b2b20218538