Prevent sync from deleting existing non-git-sources when in cog.

Bug:b/353461856
Change-Id: I08e6a27fdbcc445629898422ee04449376911a0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/5713190
Commit-Queue: Joanna Wang <jojwang@chromium.org>
Reviewed-by: Stephanie Kim <kimstephanie@google.com>
Auto-Submit: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Stephanie Kim <kimstephanie@google.com>

gclient.py

@@ -840,7 +840,9 @@ class Dependency(gclient_utils.WorkItem, DependencySettings):
 
                 # Check if at least one object needs to be downloaded.
                 needs_download = any(gcs.IsDownloadNeeded() for gcs in gcs_deps)
-                if needs_download and os.path.exists(gcs_deps[0].output_dir):
+                # When IsEnvCog(), gcs sources are already present and are not managed by gclient.
+                if not gclient_utils.IsEnvCog(
+                ) and needs_download and os.path.exists(gcs_deps[0].output_dir):
                     # Since we don't know what old content to remove, we remove
                     # the entire output_dir. All gcs_deps are expected to have
                     # the same output_dir, so we get the first one, which must