This CL factors EditableString out of EditableText and implements more of the
InputConnection functions. As a result, EditableText now basically works.

This CL also paves the way to make EditableText stateless by using
EditableString as its state object. However, there's still a bit more work to
do to make that a reality (e.g., factoring out the cursor blink timer and the
connection to the keyboard).

R=eseidel@chromium.org

Review URL: https://codereview.chromium.org/995073002