[SVG] Prevent ClipperData removal while the content is being drawn.

RenderSVGResourceClipper::postApplyResource() requires access to the
associated ClipperData to figure whether mask layers need to be
restored.

If (for whatever reason) removeClientFromCache() is triggered between
applyResource() & postApplyResource(), we need to defer the actual
removal until postApplyResource is done.

BUG=290155
R=pdr@chromium.org,schenney@chromium.org,inferno@chromium.org

Review URL: https://codereview.chromium.org/23619042

git-svn-id: svn://svn.chromium.org/blink/trunk@158761 bbb929c8-8fbe-4397-9dbb-9b2b20218538