Bug 1665648 - Initialize GPU process config before WebRender. r=kats

We need to initialize the GPU process config before WebRender because we
have a dependency on the GPU process to enable WebRender. Without this
patch, it gives very confusing initialization failures, which it should
be clear that it was disabled because there is no GPU process. The GPU
process feature itself will then have a clear reason why it was
disabled, typically because Windows did not support remote windowing
(e.g. too old).

Differential Revision: https://phabricator.services.mozilla.com/D90564