Oilpan: allocateExpandedVectorBacking() should use expandedVectorBackingHeap()
Oilpan: allocateExpandedVectorBacking() should use expandedVectorBackingHeap()

Currently both allocateVectorBacking() and allocateExpandedVectorBacking()
use vectorBackingHeap(). This is wrong (though it will just affect performance).
allocateExpandedVectorBacking() should use expandedVectorBackingHeap().

BUG=420515

Review URL: https://codereview.chromium.org/1210033004

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@198261 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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