For large object allocations, do not clear already zeroed memory.
For large object allocations, do not clear already zeroed memory.

The page memory regions are allocated with OS calls that already 
guarantee that the pages returned are all zeroes.

R=haraken
BUG=420515

Review URL: https://codereview.chromium.org/827423002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@187807 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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