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Shut down the scheduler before terminating the heap (i.e.,
PartitionAlloc). This is needed because the scheduler makes heap
allocations that need to be freed at destruction time.

BUG=391005

Review URL: https://codereview.chromium.org/475033002

git-svn-id: svn://svn.chromium.org/blink/trunk@180295 bbb929c8-8fbe-4397-9dbb-9b2b20218538