Theres a lot of good stuff in there..
Just get: src/base, src/ipc, src/net (and something else you are interested in) and you are good to go
https://www.chromium.org/developers
Then, if you need more, you will have to do it the "hacker way", by reading the chromium source code.
But hey.. you are a C++ dev, source code should be your stack overflow, and primary source of "documentation".
hint: The unit test files, can make the "how to use this" much more obvious
Also it can break any time.
Now, if you want more functionality than that.. like access to the last blink/webkit api, than you will suffer a little bit.
Theres a lot of good stuff in there..
Just get: src/base, src/ipc, src/net (and something else you are interested in) and you are good to go