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Once nacl_irt_icache has been included in irt.h in the NaCl
repository, Chrome build would fail due to multiple definition error.
This is a change for preparing for the NaCl roll that includes
the NaCl side change.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4033

Review URL: https://codereview.chromium.org/924783003

Cr-Commit-Position: refs/heads/master@{#316461}