[Fuchsia] Implement thread priorities.

[eieio]

Implement thread priorities using the Fuchsia media profile provider API to enable system / product configuration of host threads.

This change helps to address b/275114261.

[flutter-dashboard]

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[eieio]

This PR primarily changes Engine::CreateThreadHost, which is tested. However, the changes are only functional on Fuchsia in an environment configured to prioritize the Flutter threads. IIUC, the automated Flutter tests do not provide such an environment. We have adequate coverage in our Fuchsia product tests that utilize this functionality.

[chinmaygarde]

@eieio Can you suggest a reviewer? If not, I can RSLGTM.

[studgeek]

+1 based on general approach which we have used in many places for Fuchsia. Needs Flutter-specific review and approval.

[studgeek]

@eieio Can you suggest a reviewer? If not, I can RSLGTM.

LGTM very welcome. This is a pretty urgent change for a release blocking bug. Thanks!

[eieio]

@eieio Can you suggest a reviewer? If not, I can RSLGTM.

@arbreng provided the guidance internally on where this code should live, but he's out until next week.

Given that and the time constraints and LGTM would be appreciated.

[flutter-dashboard]

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Changes reported for pull request #40970 at sha 5f662c2

[eieio]

@flutter-dashboard: I don't believe you. This change only affects thread priorities and in any tests here they should be a no-op entirely.

[eieio]

Updated based on offline conversation about the platform thread being important to input latency. This final change should be good to go!

[flutter-dashboard]

Golden file changes are available for triage from new commit, Click here to view.

Changes reported for pull request #40970 at sha 60dfaf7

[eieio]

Thanks, everyone!