Make requestAdapter powerPreference optional

[beaufortfrancois]

For sake of simplicity and readability, I'd suggest making requestAdapter powerPreference optional.
It means web developers who simply want to have access to the default adapter would go with

const adapter = await gpu.requestAdapter();

instead of

const adapter = await gpu.requestAdapter({ powerPreference: 'default' });

[Kangz]

Approved in the 2018-12-03 meeting.

[Kangz]

Thanks for the contribution @beaufortfrancois !