Fuchsia: Perform script-relative path resolution for manifest files.

A previous CL regressed this behavior, leading to the generated runner
being generated with absolute paths in the buildbot's filesystem.

Change-Id: I16aaca1556c9f8eccc81d3cdd841b4a3c71de3f2
Reviewed-on: https://chromium-review.googlesource.com/978639Reviewed-by: Sergey Ulanov <sergeyu@chromium.org>
Commit-Queue: Kevin Marshall <kmarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545605}

build/fuchsia/runner_v2/create_runner_script.py

@@ -55,6 +55,7 @@ def main(args):
   group = parser.add_argument_group('Test runner path arguments.')
   group.add_argument('--output-directory')
   group.add_argument('--package')
+  group.add_argument('--package-manifest')
   args, runner_args = parser.parse_known_args(args)
 
   def RelativizePathToScript(path):
@@ -70,6 +71,8 @@ def main(args):
       ('--output-directory', RelativizePathToScript(args.output_directory)))
   runner_path_args.append(
       ('--package', RelativizePathToScript(args.package)))
+  runner_path_args.append(
+      ('--package-manifest', RelativizePathToScript(args.package_manifest)))
 
   with open(args.script_output_path, 'w') as script:
     script.write(SCRIPT_TEMPLATE.format(