Merge lp://staging/~abreu-alexandre/webbrowser-app/webapp-properties-ua-override-fix-rtm-14.09 into lp://staging/webbrowser-app/rtm-14.09

A Webapp has the option to either specify a command line option to have a specific UA overriding the default or hgave a local webapp-properties.json file that defines the UA override such as:

{
    "name": "MyWebapp",
    "includes":["http://user-agent.me/*"],
    "domain":"user-agent.me",
    "homepage":"http://user-agent.me/",
    "user-agent-override": "Mozilla/5.0 custom-user-agent-string (unlike AnyOtheBrowser)"
}

This currently does not work, or to be more precise works but the url is set before during the init flow, and the UA string is being set after the website was browsed to.

Backport of

https://code.launchpad.net/~abreu-alexandre/webbrowser-app/fix-webapp-properties-ua-override/+merge/238991

Here is the description of the MR replicated:

"A Webapp has the option to either specify a command line option to have a specific UA overriding the default or hgave a local webapp-properties.json file that defines the UA override such as:

{
    "name": "MyWebapp",
    "includes":["http://user-agent.me/*"],
    "domain":"user-agent.me",
    "homepage":"http://user-agent.me/",
    "user-agent-override": "Mozilla/5.0 custom-user-agent-string (unlike AnyOtheBrowser)"
}

This currently does not work, or to be more precise works but the url is set before during the init flow, and the UA string is being set after the website was browsed to."