Design doc: Generic Sensors WebDriver implementation

[Raphael Kubo da Costa]

Although this work does not affect Chromium's critical path, it does
touch several layers (Blink, content, chromedriver and services).

reillyg@ has already taken a first pass at the doc, and all changes are
ready for Chromium, WPT and the spec.

I'll also send an Intent email to blink-dev.

https://docs.google.com/document/d/1JS2Wnyc9GiS_u1Ry3w4KsCyd51g8fm4RDEUcanZgzbM/edit?usp=sharing