Having the device pixel ratio of the test shell be 1.0x makes it
unlikely that we will catch errors relating to mishandling of the
device pixel ratio in the test shell.
This patch arbitrarily increases the device pixel resolution while
keeping the logical resolution the same.
* 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.
