Reland "Add possibility to expand strings in the autoroller."

This is a reland of bfb6124a73af8c9fb806d9798c21aa28d36b125f

Reland with no changes, the revert was just a test.

Original change's description:
> Add possibility to expand strings in the autoroller.
>
> Starting from crrev.com/c/2265498, the Chromium DEPS file has a new
> function Str(), which seems to just expand to the string it wraps.
>
> This causes the following error:
>
>   NameError: name 'Str' is not defined
>
> In //tools_webrtc/autoroller/roll_deps.py.
>
> This CL adds a way to expand the string.
>
> No-Try: True
> Bug: None
> Change-Id: I4cdb43410edeed72b393f200314c0ee7eea9cb2a
> Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178865
> Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
> Reviewed-by: Jeremy Leconte <jleconte@webrtc.org>
> Cr-Commit-Position: refs/heads/master@{#31661}

TBR=jleconte@webrtc.org

No-Try: True
Bug: None
Change-Id: I616f7ef2ef68f9a9cb6c5e2ca332d860a750b239
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/178900
Commit-Queue: Mirko Bonadei <mbonadei@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#31677}

tools_webrtc/autoroller/roll_deps.py

@@ -97,6 +97,10 @@ class RollError(Exception):
   pass
 
 
+def StrExpansion():
+  return lambda str_value: str_value
+
+
 def VarLookup(local_scope):
   return lambda var_name: local_scope['vars'][var_name]
 
@@ -104,6 +108,7 @@ def VarLookup(local_scope):
 def ParseDepsDict(deps_content):
   local_scope = {}
   global_scope = {
+    'Str': StrExpansion(),
     'Var': VarLookup(local_scope),
     'deps_os': {},
   }