Audit navigator.gpu

We have a new fancy navigator property: navigator.gpu.

We need to understand what it is reporting and whether it's a fingerprinting hazard.

Quick test on MB nightly:

>> Array.from(navigator.gpu.wgslLanguageFeatures.entries())
Array(3) [ (2) […], (2) […], (2) […] ]
​    0: Array [ "readonly_and_readwrite_storage_textures", "readonly_and_readwrite_storage_textures" ]
​    1: Array [ "packed_4x8_integer_dot_product", "packed_4x8_integer_dot_product" ]
​    2: Array [ "pointer_composite_access", "pointer_composite_access" ]
​    length: 3