[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

Review-Url: https://codereview.chromium.org/2296043004
Cr-Original-Commit-Position: refs/heads/master@{#415753}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: 9f0f2b6fa647284df16706073cd7b916ab9f6af3