Re-enable Chrome testing on CI

[alexcrichton]

I'm about to send a PR to disable it as it's been very flaky and we haven't had the time to bottom it out. We should track down the issue. Some example failing logs are:

• https://travis-ci.org/rustwasm/wasm-bindgen/jobs/432608027
• https://travis-ci.org/rustwasm/wasm-bindgen/jobs/431715986
• https://travis-ci.org/rustwasm/wasm-bindgen/jobs/430725910
• https://travis-ci.org/rustwasm/wasm-bindgen/jobs/430312532

It's unclear whether this is a bug in our webdriver implementation or in Chrome itself...

[xv-matt-u]

Is this still relevant since #1300?

[alexcrichton]

This still yes, yeah, in the sense that we're still only running tests in headless Firefox, not in Chrome yet.