DevTools: [Console] fix selection of all console messages

It turns out that selection of all elements in console (aka Ctrl-A)
works properly only if the last element in viewport DOM is a block
element with some text.

BUG=410899
R=vsevik

Review URL: https://codereview.chromium.org/564893002

git-svn-id: svn://svn.chromium.org/blink/trunk@181850 bbb929c8-8fbe-4397-9dbb-9b2b20218538