DevTools: flatten console panel messages structure

Console API has methods console.groupStart() and console.groupEnd()
which make it possible to wrap console messages. It is allowed to
have nested console.groupStart() calls, which implies tree-like
structures.

At present, group hierarchical structures were mirrored via DOM
nested elements, which made it complicated to implement a viewporting
in console.

This patch supports future attemp in viewport implementation. It removes
DOM nesting elements and makes all messages to be stored as a properly
decorated siblings next to each other.

R=sergeyv, pfeldman

Review URL: https://codereview.chromium.org/250893003

git-svn-id: svn://svn.chromium.org/blink/trunk@172651 bbb929c8-8fbe-4397-9dbb-9b2b20218538