[DevTools] Various tweaks to inspector_protocol.
- allow importing all domains in single header;
- support for <includes.h>;
- separate export macro/header for lib, protocol and exported;
- improved code for defaults.
BUG=
637032
Committed:
https://crrev.com/9f0f2b6fa647284df16706073cd7b916ab9f6af3
Cr-Commit-Position: refs/heads/master@{#415753}