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DiscardVirtualMemory makes the contents of discarded memory undefined.
c.f. https://msdn.microsoft.com/ja-jp/library/windows/desktop/dn781432(v=vs.85).aspx

BUG=

Change-Id: Ifd06e6f75271dc6a38cf0a34027cb20f1a887cfe
Reviewed-on: https://chromium-review.googlesource.com/571621
Commit-Queue: Takashi Sakamoto <tasak@google.com>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#486700}