[fuchsia] Modify manifest generation to use partial CMX files.

* Modify all the *sandbox_policy files to be partial CMX files and
  rename them to <package_name>.cmx
* Modify the final CMX generation to merge in the package-supplied
  partial CMX file rather than treat its content as the "sandbox"
  section of the final CMX.
* Rename the |sandbox_policy| parameter for the fuchsia_package GN
  template to |manifest|

Bug: 979830
Change-Id: I7dc466484139db7f62bfe6a713bcfe3ac22b1a2a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1682556
Reviewed-by: John Budorick <jbudorick@chromium.org>
Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Reviewed-by: Luke Halliwell <halliwell@chromium.org>
Reviewed-by: Sami Kyöstilä <skyostil@chromium.org>
Commit-Queue: Fabrice de Gans-Riberi <fdegans@chromium.org>
Cr-Original-Commit-Position: refs/heads/master@{#675396}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: cc810d3c0e038246977a639d22841431aff6019f
3 files changed