[WebGPU article] Check adapter before using it

[beaufortfrancois]

This PR makes sure adapter is valid before using. I've also updated all Glitch samples.
Context: https://chromium-review.googlesource.com/c/chromium/src/+/2617499

@Kangz for review

Live preview: https://pr-9198-dot-web-central.appspot.com/web/updates/2019/08/get-started-with-gpu-compute-on-the-web

[WebFundBot]

Whoops!

There were 3 warnings that will prevent this PR from being merged. Please take a look, and either fix, or provide a justification for why they can't be fixed.

WARNINGS
src/content/en/updates/2019/08/get-started-with-gpu-compute-on-the-web.md#L73 - Common typo found: 'implicity' Should it be 'implicitly'?
src/content/en/updates/2019/08/get-started-with-gpu-compute-on-the-web.md#L231 - Common typo found: 'implicity' Should it be 'implicitly'?
src/content/en/updates/2019/08/get-started-with-gpu-compute-on-the-web.md#L238 - Line must be at most 100 characters

[beaufortfrancois]

@petele Please merge and push live.