Add non_git_source value to DEPS to allowlist non-git sources.

Some `deps` entries may not be compatible with every system
that parses DEPS. This condition will be used to allowlist
deps slowly that are expected to work with every supported DEPS parser.

Bug: b/344601229
Change-Id: Idb076250f2ca4924603029e5206bcc6bc6c75231
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/353440
Commit-Queue: Jeremy Leconte <jleconte@google.com>
Reviewed-by: Jeremy Leconte <jleconte@google.com>
Auto-Submit: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/main@{#42426}

DEPS

@@ -50,6 +50,9 @@ vars = {
   # ninja CIPD package version
   # https://chrome-infra-packages.appspot.com/p/infra/3pp/tools/ninja
   'ninja_version': 'version:2@1.11.1.chromium.6',
+
+  # condition to allowlist deps for non-git-source processing.
+  'non_git_source': 'True',
 }
 
 deps = {