Fix Twitter linkcheck failure.

[markmandel]

What type of PR is this?

Uncomment only one /kind <> line, hit enter to put that in a new line, and remove leading whitespace from that line:

/kind breaking

/kind bug

/kind cleanup
/kind documentation
/kind feature
/kind hotfix

What this PR does / Why we need it:

Twitter seems to be doing UserAgent sniffing, and rejecting non-browser user agents, breaking CI.

Which issue(s) this PR fixes:
N/A

Special notes for your reviewer:

This change makes htmltest use a Chrome Linux user agent to work around this issue. To support editing the HTTP headers required an update to htmltest.

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: markmandel, roberthbailey

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Needs approval from an approver in each of these files:

• OWNERS [markmandel,roberthbailey]

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[agones-bot]

Build Succeeded 👏

Build Id: c8920c38-76b3-48d2-b99b-e96cb0d476e6

The following development artifacts have been built, and will exist for the next 30 days:

• image: gcr.io/agones-images/agones-controller:1.12.0-23494dc
• image: gcr.io/agones-images/agones-sdk:1.12.0-23494dc
• image: gcr.io/agones-images/agones-ping:1.12.0-23494dc
• Linux C++ SDK (build): agonessdk-1.12.0-23494dc-linux-arch_64.tar.gz
• SDK Server: agonessdk-server-1.12.0-23494dc.zip

A preview of the website (the last 30 builds are retained):

• https://23494dc-dot-preview-dot-agones-images.appspot.com/

To install this version:

• git fetch https://github.com/googleforgames/agones.git pull/1947/head:pr_1947 && git checkout pr_1947
• helm install ./install/helm/agones --namespace agones-system --name agones --set agones.image.tag=1.12.0-23494dc