Clear baseRenderStyle on detach.

ActiveAnimations::baseRenderStyle was only nulled correctly from
styleForElement, but that method is not always called when clearing the
styleChangeType(). For instance when the we have detached the element, but
don't need to create a renderer for it when re-attaching.

This bug caused the baseRenderStyle to survive moving an element between
documents triggering the assert checking that the baseRenderStyle was
correct.

Clearing the baseRenderStyle unconditionally on detach() should fix this.

R=dstockwell@chromium.org,esprehn@chromium.org
BUG=427433

Review URL: https://codereview.chromium.org/642773004

git-svn-id: svn://svn.chromium.org/blink/trunk@184515 bbb929c8-8fbe-4397-9dbb-9b2b20218538