Print a BUILD_STEP marker for spec2k bot's "build prerequisites step"
Print a BUILD_STEP marker for spec2k bot's "build prerequisites step"

Without this, errors in this step show up on the waterfall as being
in the "gclient runhooks" step.

BUG= none
TEST= none

R=jvoung@google.com,pdox@google.com

Review URL: http://codereview.chromium.org/8672003

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@7267 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client