Tweak handling of branch prediction mitigations

PROCESS_CREATION_MITIGATION_POLICY2_RESTRICT_INDIRECT_BRANCH_PREDICTION_ALWAYS_ON
first shows up after the 16299 SDK but we need to use it immediately.
Now that the 10.0.17133.0 SDK has shipped we can define it in such a
way that Chrome will cleanly build with or without it.

Once Chrome requires the 17133 SDK or later we can remove our local
define of it.

Bug: 808526
Change-Id: I964a75616191e98acf0d7d3c4f570de5c6d9f15a
Reviewed-on: https://chromium-review.googlesource.com/1006397
Reviewed-by: Penny MacNeil <pennymac@chromium.org>
Commit-Queue: Bruce Dawson <brucedawson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#549939}
1 file changed