Bump up instance class of chromium-try-flakes.
Bump up instance class of chromium-try-flakes.

This was set before in the console, but looks like it needs to be specified in a file now.

Review URL: https://codereview.chromium.org/1345743002

nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra,nicko96/Chrome-Infra