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With the Speech API now using Oilpan by default, follow up and also
heap allocate the platform-level intermediary objects.

R=tkent@chromium.org,haraken@chromium.org
BUG=340522

Review URL: https://codereview.chromium.org/339823003

git-svn-id: svn://svn.chromium.org/blink/trunk@176321 bbb929c8-8fbe-4397-9dbb-9b2b20218538