The PlatformView superclass constructor was posting a task to the UI thread
that adds the view to the shell's global list. This could result in UI thread
operations seeing PlatformView instances that are not fully constructed and do
not yet have an engine.
This was happening in https://github.com/flutter/flutter/issues/7735
* Namespaces have been updated to reflect the move from //flutter/sky/shell to //flutter/shell.
* shell/BUILD.gn file has been split into smaller GN files for each subcomponent of the shell (common, GPU, diagnostic, testing).
* GN dependencies have been rewritten to stop exposing common shell dependencies as public. Duplicates have also been removed.
* GPU subcomponent has been updated make it more suitable for Vulkan integration.
* The GLFW backend has been resurrected.
