Updating Chromium to a new Fuchsia SDK

1. Check the Fuchsia-side job for a recent green archive. On the “SUCCEEDED” link, copy the SHA-1 from the gsutil.upload link of the upload fuchsia-sdk step.
2. Put that into Chromium's src.git DEPS in the fuchsia_sdk step.
3. gclient sync && ninja ... and make sure things go OK locally.
4. Upload the roll CL, making sure to include the fuchsia trybot. Tag the roll with Bug: 707030.

(Old revisions of this document have the old manual steps for building a Fuchsia SDK if for some reason you want to do that locally. They'll probably increasingly go out of date as the steps for building the SDK changes though.)