ChromeOS: Add "Safely ignore this error in VMTests" message to InitializeSandbox() error message.
ChromeOS: Add "Safely ignore this error in VMTests" message to InitializeSandbox() error message.

Chrome on VMs uses a GPU driver that creates threads, the GPU sandbox will warn when this happens,
because it means the sandbox is unsafe. So it's expected that we get the InitializeSandbox()
error message. This error message shows up for every VMTest, which might make it difficult
to identity actual errors.

BUG=585511

Review-Url: https://codereview.chromium.org/1882243002
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#390120}

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