Add a powerPreference option

[greggman]

Not sure how to add this here's an attempt.

You can pass powerPreference=low-power or
power-preference=high-performance in the query string.

Issue: #2083

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Requirements for PR author:

• All missing test coverage is tracked with "TODO" or .unimplemented().
• New helpers are /** documented */ and new helper files are found in helper_index.txt.
• Test behaves as expected in a WebGPU implementation. (If not passing, explain above.)

Requirements for reviewer sign-off:

• Tests are properly located in the test tree.
• Test descriptions allow a reader to "read only the test plans and evaluate coverage completeness", and accurately reflect the test code.
• Tests provide complete coverage (including validation control cases). Missing coverage MUST be covered by TODOs.
• Helpers and types promote readability and maintainability.

When landing this PR, be sure to make any necessary issue status updates.

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[kainino0x]

LGTM, see comment about possible alternative

[kainino0x]

Normally I'd avoid interposing on the API since it means tests aren't running against the implementation itself, but it's probably fine since it only happens when you explicitly enable it. There are some tests which this will have slightly surprising effects on though, specifically device_allocation.spec.ts and requestAdapter.spec.ts.

The other option would be to change the requestAdapter call in device_pool.ts, which would affect most tests but not the few that call requestAdapter themselves.

[greggman]

So I looked into adding it to device_pool.ts and it wasn't clear to me how to make it work. First I tried adding a requestAdapterOptions to DevicePool and that needed to be passed down to DescriptorToHolderMap and then to DeviceHolder. Then I tried pass in a default at the top where it creates the device pool singleton. But, you obviously can't have a function to set the defualt in that file since the function needs to be called before the singleton is created.

I put the functions to set (and get) the default options in a separate file alongsite device_pool.ts but when I tried to call it from standalone.ts I got an error calling into that part of the tree is off-limits.

Maybe there's an easier way but for now I decided to leave it as is. I'm happy to refactor into device_pool.ts with some guidance on how to do it.

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