Holding the copy modifier during a drag should copy text.

Original patch by erceg.david@gmail.com.
The current code is busted, as we can't read key state from the renderer
process. Instead, just use the key state flags that are passed in the
event.

BUG=49639

Review URL: https://codereview.chromium.org/181823003

git-svn-id: svn://svn.chromium.org/blink/trunk@168151 bbb929c8-8fbe-4397-9dbb-9b2b20218538