[iOS] Crash in WebCore::Node::renderRect

[iOS] Crash in WebCore::Node::renderRect
https://bugs.webkit.org/show_bug.cgi?id=196035
<rdar://problem/49076783>

Reviewed by Antoine Quint.

When renderRect was called on an HTMLAreaElement, it would
ASSERT because it doesn't have a renderer. We hadn't noticed
this before because none of our tests were hitting this in
debug mode.

The fix is to ask the corresponding HTMLImageElement for
its renderer, and use that for the returned rectangle.

Covered by these tests that had become flakey:
    fast/images/imagemap-in-shadow-tree.html
    http/tests/download/area-download.html

* dom/Node.cpp:
(WebCore::Node::renderRect):

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@243249 268f45cc-cd09-0410-ab3c-d52691b4dbfc