Enable build variants

[John-LittleBearLabs]

The idea is to ease the dev process of downstream variants of Electron, which add built-in functionality which might be seen as too specific or out-of-scope for upstream... without maintaining patches for Electron code itself. If the idea is accepted in principle, I intend to contribute toward this end.

For example (obviously the names here are negotiable/bikesheddable):
electron/BUILD.gn uses exec_script, which...

• If ../electron-spin/BUILD.gn is absent Electron builds exactly as it does now

• Otherwise it gains a dependency on //electron-spin:hooks

  • For each X in a list defined in the script, if ../electron-spin/X.h exists a define is added to Electron

• In a few strategic places in the code, #ifdef those defines, call some function whose name is effectively API.

  • For example, ElectronBrowserClient::WillCreateURLLoaderRequestInterceptors might call CreateSpinRequestInterceptors, but only if ELECTRON_SPIN_INTERCEPTORS is defined, which would happen if ../electron-spin/interceptors.h exists.

  I could also use __has_include instead of defines if you prefer to take that part out of the build system, but the exec_script would still be useful to have the dependency if and only if someone did actually add their directory there.

  Does this seem like a reasonable approach?

[John-LittleBearLabs]

Personally, my main interest would be related to this , but the hope is to make the hooks generic enough that it could be useful for various features.