Refactor RenderQuote to not delete renderers on style change.

Currently the RenderQuote code will destroy all child nodes when the style
changes. This CL updates the code to re-use the existing renderer instead
of re-creating.

This fixes an issue with FirstLetterPseudoElement where we were destroying
the first-letter renderers out from under the elements feet.

BUG=424914

Review URL: https://codereview.chromium.org/679593004

git-svn-id: svn://svn.chromium.org/blink/trunk@184397 bbb929c8-8fbe-4397-9dbb-9b2b20218538