Avoid Node references from AccessibilityImageMapLink from leaking.

AccessibilityImageMapLink has two RefPtrs, m_areaElement and m_mapElement.
However, they were not cleared when an AccessibilityImageMapLink instance detached.

BUG=270000

Review URL: https://chromiumcodereview.appspot.com/22893033

git-svn-id: svn://svn.chromium.org/blink/trunk@156545 bbb929c8-8fbe-4397-9dbb-9b2b20218538