[Fuchsia] Add CIPD package specification for the CastRunner archive.

Bug: 893229
Change-Id: I509c18dcc30c7962bd5a496396556dd8cbf9af06
Reviewed-on: https://chromium-review.googlesource.com/c/1318578Reviewed-by: James Robinson <jamesr@chromium.org>
Reviewed-by: Kevin Marshall <kmarshall@chromium.org>
Commit-Queue: Wez <wez@chromium.org>
Cr-Commit-Position: refs/heads/master@{#605486}

webrunner/cipd/castrunner.yaml

+# Copyright 2018 The Chromium Authors. All rights reserved.
+# Use of this source code is governed by a BSD-style license that can be
+# found in the LICENSE file.
+
+# Creates a package for the Cast application Runner.
+# Note that the web.ContextProvider package is required by the Cast Runner.
+# The ID of the target architecture (amd64, arm64) must be passed in as
+# a pkg-var so that the packages can be archived at the appropriate location
+# in the CIPD package hierarchy.
+#
+# pkg-var arguments:
+#   outdir: A fully qualified path to the build output directory.
+#   targetarch: The target architecture, either "amd64" or "arm64".
+#
+# To create a CIPD package, run the following command from the build output
+# directory.
+#
+# $ cipd create --pkg-def ../../webrunner/cipd/castrunner.yaml \
+#               -pkg-var targetarch:$TARGET_ARCH \
+#               -pkg-var outdir:`pwd` \
+#               -ref latest \
+#               -tag version:$(cat fuchsia_artifacts/build_id.txt)
+#
+# The most recent package can be discovered by searching for the "latest" ref:
+#
+# $ cipd describe chromium/fuchsia/castrunner-$TARGET_ARCH -version latest
+
+package: chromium/fuchsia/castrunner-${targetarch}
+description: Prebuilt Cast application Runner binaries for Fuchsia.
+root: ${outdir}/fuchsia_artifacts
+data:
+  - file: cast_runner.far
+  - file: LICENSE