gclient flatten: fix an issue with custom_deps

TBR=dpranke

Bug: 570091
Change-Id: I1b8cc8156c83c72b2fc599d90059bc7f2674ca17
Reviewed-on: https://chromium-review.googlesource.com/532995Reviewed-by: Paweł Hajdan Jr. <phajdan.jr@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>

gclient.py

@@ -561,6 +561,9 @@ class Dependency(gclient_utils.WorkItem, DependencySettings):
 
   def _postprocess_deps(self, deps, rel_prefix):
     """Performs post-processing of deps compared to what's in the DEPS file."""
+    # Make sure the dict is mutable, e.g. in case it's frozen.
+    deps = dict(deps)
+
     # If a line is in custom_deps, but not in the solution, we want to append
     # this line to the solution.
     for d in self.custom_deps: