buildbot_valgrind: Move clobber step before gclient runhooks
buildbot_valgrind: Move clobber step before gclient runhooks

The step that clears out local state including the toolchain directory
should come before the step that's meant to populate that directory.

BUG=none
TEST=none

R=ncbray@google.com
TBR=

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@5869 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

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