The files required in a service process can now be listed in the service's
manifest.
In the manifest the file path is given with a string ID and a platform. The file
is then opened in the parent process and passed to the child process, its ID is
specified with a new command-line switch that associates the manifest specified
key and a GlobalDescriptor ID. There are used to populate the new
FileDescriptorStore which is similar to GlobalDescriptor but uses string as keys
(which makes it easier to prevent ID collisions between multiple services).
The service code can then retrieve the FD from the FileDescriptorStore.
The goal is to eventually switch entirely to only using the FileDescriptorStore,
and getting rid of the GlobalDescriptor.

BUG=690189

Review-Url: https://codereview.chromium.org/2684433003
Cr-Commit-Position: refs/heads/master@{#451019}