Windows SDK update to 28000

[Devon Loehr]

Action required: if you build chromium without Google's bundled windows toolchain, you'll need to update your local SDK to the most recent version. Googlers building internally don't need to do anything.

We're currently working on updating the windows SDK we use when building chromium from 10.0.26100.* to 10.0.28000.2270. This is a major version bump, so the chance for something to go wrong is higher, but it's survived a few days.

As usual, once the SDK has been stable for ~2 weeks, we'll drop support for the previous version. Therefore, everyone who builds chromium will need to ensure they've updated their SDK by that point. For Googlers, the process is automatic, and `gclient sync` will pick up the newest changes.

Because SDK rolls can cause difficult-to-detect problems, we ask that developers refrain from using new SDK features for those 2 weeks until we've settled on the new SDK version. Otherwise it will be difficult to roll back if something should go wrong later.

--Devon Loehr