Fix handling of case where the same AccessibilityObject appears as a

      child of more than one parent. Add some additional DCHECKs so that such
a problem would be caught even earlier if they were to reoccur.

BUG=75496
TEST=Addes new browser test CrossPlatformMultipleInheritanceAccessibility2

Review URL: http://codereview.chromium.org/6683010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@78025 0039d316-1c4b-4281-b951-d872f2087c98