Chrome DevTools CSS Domain

Implement the CSSAgent, part of the Chrome Debugging Protocol, inside Chrome DevTools.

Features:

• Preview CSS styles used in application/page
• Preview computed styles
• Preview inline styles

Requirements:

ping @vakrilov @hshristov @PanayotCankov

In order to display proper styles information, the CSS agent needs to be sent information following a very specific interface for the selected node in the visual tree.

Selecting an element from the DOM requires the following information to be computed and sent to the inspector:

• Inline styles -> CSSStyle
• Style matches -> RuleMatch[]
• Inherited styles -> InheritedStyleEntry[]
• (out of the 3.1 scope) Pseudo Element matches -> PseudoElementMatches[]
• Computed Styles for node -> CSSComputedStyleProperty[]
• (out of the 3.1 scope) Element Fonts -> PlatformFontUsage

CSSStyle, RuleMatch, InheritedStyleEntry, PseudoElementMatches, CSSComputedStyleProperty, PlatformFontUsage classes' definitions can be previewed at https://github.com/NativeScript/android-runtime/blob/43f9bd0ff7cc96e0560bf5f965ee5f01e19f12d5/test-app/app/src/main/assets/app/css-classes.d.ts#L3

Part of Epic #563

先从 Chrome Debugging Protocol CSSAgent 文档以及 issue 中链接的 css-classes.d.ts 定义开始，然后检查现有的 computed-style preview 和 visual-tree node selection path。当剩余的、属于范围内的 CSSAgent 样式信息都暴露给 inspector，同时 pseudo-element matches 和 element fonts 仍不在范围内时，工作即告完成。