[fuchsia] Unconditionally register fuchsia-out-dir arg in runner script.

Allows fuchsia-out-dir to be set independent of whether it will actually
be consumed by the target type. Targets which don't use external
Fuchsia outdirs, e.g. "qemu" and "aemu", simply ignore the arg.

Otherwise, the arg parsing step is broken when the
"default_fuchsia_build_dir_for_installation" GN arg is set.
This is a likely event on x64 builds which run equally well on emulators
and baremetal hardware.

Bug: 1080854
Change-Id: I4690737a36f7d60c9992421a8752caeffed9084f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2402100Reviewed-by: Fabrice de Gans-Riberi <fdegans@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810908}

build/fuchsia/common_args.py

@@ -107,6 +107,11 @@ def AddCommonArgs(arg_parser):
   common_args.add_argument('--verbose', '-v', default=False,
                            action='store_true',
                            help='Enable debug-level logging.')
+  common_args.add_argument('--fuchsia-out-dir',
+                           nargs='+',
+                           help='Path to a Fuchsia build output directory. '
+                           'If more than one outdir is supplied, the last one '
+                           'in the sequence will be used.')
 
 
 def ConfigureLogging(args):

build/fuchsia/device_target.py

@@ -140,10 +140,6 @@ class DeviceTarget(target.Target):
                              '-F',
                              help='The path to the SSH configuration used for '
                              'connecting to the target device.')
-    device_args.add_argument('--fuchsia-out-dir',
-                             help='Path to a Fuchsia build output directory. '
-                             'Equivalent to setting --ssh_config and '
-                             '--os-check=ignore')
     device_args.add_argument(
         '--os-check',
         choices=['check', 'update', 'ignore'],