set up Travis CI

Our Chromium-infrastructure buildbots are a pain to maintain with all of the Chromium infrastructure changes. The proposal is to switch to a combination of distributed developer-owned machines submitting to CDash as well as setting up Travis CI for continuous integration testing. Travis supports Linux and most importantly Mac, to give us coverage there. Windows, ARM, and AArch64 will have to rely solely on CDash.

For nightly public builds we have not yet decided on a final solution.