Use the same fake User-Agent string as GetContentShell()->GetUserAgent() for URLReque...

Use the same fake User-Agent string as GetContentShell()->GetUserAgent() for URLRequestContext in content_shell

ShellContentClient::GetUserAgent() returns a fake User-Agent. It's used
for navigator.userAgent, loading layout test test case file, etc. But
ShellURLRequestContextGetter has an HttpUserAgentSettings with an empty
string for User-Agent. Factor out the code to build the fake User-Agent
from ShellContentClient::GetUserAgent() into GetShellUserAgent() and use
it for ShellURLRequestContextGetter. This is almost the same as how
ChromeShellContentClient and ChromeURLRequestContextGetter are doing.

With this, we can test that WebSocket handshake request contains the
correct user-agent header value.

BUG=347109

Review URL: https://codereview.chromium.org/319083003

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@275673 0039d316-1c4b-4281-b951-d872f2087c98