DevTools: [CodeMirror] fix coordinates-to-position implementation

This patch fixes the pixel coordinates to text position function
which is responsible for the popover target.

BUG=265521

Review URL: https://chromiumcodereview.appspot.com/21104004

git-svn-id: svn://svn.chromium.org/blink/trunk@155160 bbb929c8-8fbe-4397-9dbb-9b2b20218538