Windows keeps track of files downloaded from the Internet and powershell by
default refuses to execute them (even with RemoteSigned policy). We need to
explicitly unblock the file first.

Note: it requires Powershell >= 3.0.

R=dpranke@chromium.org, iannucci@chromium.org
BUG=663843

Change-Id: Id681f4058f906fa4782a360be184d132d837ae78
Reviewed-on: https://chromium-review.googlesource.com/424327Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Vadim Shtayura <vadimsh@chromium.org>