Make proxy settings one atomic dictionary in the PrefStores such that modifications are atomic.

chrome/common/extensions/api/extension_api.json

Index: chrome/common/extensions/api/extension_api.json
diff --git a/chrome/common/extensions/api/extension_api.json b/chrome/common/extensions/api/extension_api.json
index 9f6deff54b909009bd8186bcad9a5c34039179fc..4fc7492dc31c881e5cc682518c7d23d800ae3337 100644
--- a/chrome/common/extensions/api/extension_api.json
+++ b/chrome/common/extensions/api/extension_api.json
@@ -3743,7 +3743,7 @@
       {
         "id": "ProxyRules",
         "type": "object",
-        "description": "An object encapsulating the set of proxy rules for all protocols.",
+        "description": "An object encapsulating the set of proxy rules for all protocols. Use either 'singleProxy' or (a subset of) 'proxyForHttp', 'proxyForHttps', 'proxyForFtp' and 'socksProxy'.",
         "properties": {
           "singleProxy": {"$ref": "ProxyServer", "optional": true, "description": "The proxy server to be used for all per-URL requests (i.e., http, https, and ftp)."},
           "proxyForHttp": {"$ref": "ProxyServer", "optional": true, "description": "The proxy server to be used for HTTP requests."},
@@ -3765,8 +3765,8 @@
         "type": "object",
         "description": "An object encapsulating a complete proxy configuration.",
         "properties": {
-          "rules": {"$ref": "ProxyRules", "optional": true, "description": "The proxy rules describing this configuration."},
-          "pacScript": {"$ref": "PacScript", "optional": true, "description": "The proxy auto-config (PAC) script for this configuration."},
+          "rules": {"$ref": "ProxyRules", "optional": true, "description": "The proxy rules describing this configuration. Use this for 'fixed_servers' mode."},
+          "pacScript": {"$ref": "PacScript", "optional": true, "description": "The proxy auto-config (PAC) script for this configuration. Use this for 'pac_script' mode."},
           "mode": {
             "type": "string",
             "enum": ["direct", "auto_detect", "pac_script", "fixed_servers", "system"],