Pause SVG animations when the document is detached.

This patch pauses SVG animations if the document is detached or
disposed (matching m_scriptedAnimationController's behavior). SVG 
animations will already reset when the document is re-attached so there
are no additional unpause() calls.

This patch allows us add an assert for document()->frame() in
StyleResolver::styleForElement. No new tests as this functionality is
covered with existing tests and the new assert.

BUG=260966

Review URL: https://chromiumcodereview.appspot.com/19525002

git-svn-id: svn://svn.chromium.org/blink/trunk@154382 bbb929c8-8fbe-4397-9dbb-9b2b20218538