[Extensions] Update developerPrivate API to use base-64 data urls for icons

chrome/test/data/extensions/api_test/developer/generated_output/hpiknbiabeeppbpihjehijgoemciehgk.json

 {
    "actionButtonHidden": false,
    "dependentExtensions": [  ],
    "description": "",
    "disableReasons": {
       "corruptInstall": false,
       "suspiciousInstall": false,
       "updateRequired": false
    },
    "errorCollection": {
       "isActive": false,
       "isEnabled": false
    },
    "fileAccess": {
       "isActive": false,
       "isEnabled": false
    },
    "homePage": {
       "specified": false,
       "url": ""
    },
-   "iconUrl": "chrome://extension-icon/hpiknbiabeeppbpihjehijgoemciehgk/48/1",
    "id": "hpiknbiabeeppbpihjehijgoemciehgk",
    "incognitoAccess": {
       "isActive": false,
       "isEnabled": true
    },
    "installWarnings": [  ],
    "installedByCustodian": false,
    "location": "UNKNOWN",
    "locationText": "Not from Chrome Web Store.",
    "manifestErrors": [  ],
@@ skipping 18 matching lines @@
       "type": "TAB_CONTENTS",
       "url": "chrome-extension://hpiknbiabeeppbpihjehijgoemciehgk/bar.html"
    }, {
       "incognito": false,
       "renderProcessId": 0,
       "renderViewId": 0,
       "type": "TAB_CONTENTS",
       "url": "chrome-extension://hpiknbiabeeppbpihjehijgoemciehgk/bar.html"
    } ]
 }