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The demo does GL on the main thread, which broken since we now depend on
GpuPlatformSupport(Host). Let's deliver these messages directly via a
helper class for those applications.

BUG=409978
TEST=ozone_demo --ozone-platform=gbm on link_freon
NOTRY=true

Review URL: https://codereview.chromium.org/533803003

Cr-Commit-Position: refs/heads/master@{#293075}