Add getdep functionality for GCS deps

> gclient getdep -r src/third_party/js_code_coverage
[{'object_name': 'js_code_coverage/d538975c93eefc7bafd599b50f867e90c1ef17f3', 'sha256sum': '646bb00ced0a930b2eb1e4dbcfac18ebbb8f889bb80599e0254d9d6505427914', 'size_bytes': 1469185, 'generation': 1657780123604338}]

Bug: b/328065301
Change-Id: I9dabe84751905f5cc52c861e565f9adbcf0043fd
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5506847
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Stephanie Kim <kimstephanie@google.com>

gclient_eval.py

@@ -958,4 +958,7 @@ def GetRevision(gclient_dict, dep_name):
         _, _, revision = dep['url'].partition('@')
         return revision or None
 
-    raise ValueError('%s is not a valid git dependency.' % dep_name)
+    if isinstance(gclient_dict, _NodeDict) and 'objects' in dep:
+        return dep['objects']
+
+    raise ValueError('%s is not a valid git or gcs dependency.' % dep_name)