I did some research on the same thread and tried the answers that I got. But still, the same error throws every time.
In my case, it's a chrome app and on one page I'm using an iframe and I'm pointing src into some XHTML content. That actually contain some inline style and scripts, which I cant remove or change.
Below is my manifest file.
{
    "manifest_version": 2,
    "name": "*****",
    "short_name": "****",
    "version": "*****",
    "permissions": [""],
    "content_security_policy": "default-src 'self' 'unsafe-inline' 'unsafe-eval'; script-src 'unsafe-inline' 'unsafe-eval' 'self'; object-src 'self' 'unsafe-inline' 'unsafe-eval'",
    "app": {
        "background": {
            "scripts": ["chrome.js"]
        }
    },
    "icons": {
        "16": "1.png",
        "48": "2.png",
        "128": "3.png"
        },
    "sandbox": {
        "pages": [
            "****.html"
        ]
    }
}
 
     
    