DevTools: do not use CSSMetadata instances for autocompletion
The WI.CSSMetadata should serve a single purpose: hold meta-information about CSS properties.
However, today WI.CSSMetadata also provides an API for CSS property autocompletion in
Styles sidebar and Source frame. There's no good reason for that.
BUG=none
Committed:
https://crrev.com/83b7b48f0c6c89d2325ab7fdc96917871acf8708
Cr-Commit-Position: refs/heads/master@{#410117}