Fix WebGL roll script for recent ChromeOS-specific DEPS changes.

Add an evaluator for "Str('')".

Tbr: jdarpinian@chromium.org
Tbr: shrekshao@google.com
Bug: 1097235
Change-Id: I9f35c8696215970e95d65dd476f312705fbf59d8
No-Try: True
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2318651Reviewed-by: Kenneth Russell <kbr@chromium.org>
Commit-Queue: Kenneth Russell <kbr@chromium.org>
Cr-Commit-Position: refs/heads/master@{#791467}

tools/roll_webgl_conformance.py

@@ -97,8 +97,9 @@ def _ParseDepsFile(filename):
 def _ParseDepsDict(deps_content):
   local_scope = {}
   global_scope = {
-    'Var': _VarLookup(local_scope),
-    'deps_os': {},
+      'Str': lambda arg: str(arg),
+      'Var': _VarLookup(local_scope),
+      'deps_os': {},
   }
   exec(deps_content, global_scope, local_scope)
   return local_scope