devtools_api: Fix Python 3 compatibility

itervalues() no longer exists in Python 3. values() works on both.

Traceback (most recent call last):
  File "../../headless/lib/browser/devtools_api/client_api_generator.py", line 525, in <module>
    InitializeDomainDependencies(json_api)
  File "../../headless/lib/browser/devtools_api/client_api_generator.py", line 425, in InitializeDomainDependencies
    GetDomainDepsFromRefs(domain['domain'], domain)
  File "../../headless/lib/browser/devtools_api/client_api_generator.py", line 413, in GetDomainDepsFromRefs
    for value in json.itervalues():
AttributeError: 'dict' object has no attribute 'itervalues'

Bug: 941669
Change-Id: I24a888b95ead33b9a9abfdd84f08ac5fc10b4136
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2089862
Auto-Submit: Raul Tambre <raul@tambre.ee>
Reviewed-by: Johannes Henkel <johannes@chromium.org>
Commit-Queue: Raul Tambre <raul@tambre.ee>
Cr-Commit-Position: refs/heads/master@{#747576}

headless/lib/browser/devtools_api/client_api_generator.py

@@ -410,7 +410,7 @@ def InitializeDomainDependencies(json_api):
 
     if not isinstance(json, dict):
       return
-    for value in json.itervalues():
+    for value in json.values():
       GetDomainDepsFromRefs(domain_name, value)
 
     if '$ref' in json: