The fix to prevent accidentally scrolling away from the comment dialog

broke scrolling within the text editor. This change adds some smarts so

we figure out if we're trying to scroll in the text editor and the text

editor is capable of scrolling, we'll allow it, and if not, we'll

continue to swallow the event.

Verified that the scroll wheel did not do anything when moused over

  parts of the comment dialog other than the text editor.
Verified that the scroll wheel did not do anything when moused over an

  empty text field, in both markdown and plain text modes.
Verified that the scroll wheel did not do anything when moused over

  text that did not overflow the text editor in markdown mode.
Verified that the scroll wheel scrolled the text editor when working

  with long comments in both markdown and plan text modes.
Ran js-tests.