UrlRequestFlags - Update with new values

https://bitbucket.org/chromiumembedded/cef/commits/cd5995a440302e0ef5a97a000faa922149ffb177?at=3683#Linclude/internal/cef_types.hT1281

Revert "build.ps1 - Upgrade to support new version scheme"

This reverts commit 1f39e42.

Upgrade to CEF 3.3578.1870.gc974488 / Chromium 71.0.3578.98

WindowInfo.h - Make unmanaged constructor internal

RequestContext - Improve CEF UI thread xml doc

Cef.h - Remove ThrowIfCefNotInitialized

As IsInitialized is set when CefInitialize returns and it appears to be possible the OnContextInitialized is called
before it returns then this exception wasn't going to work.

Testing with Cef.GetGlobalRequestContext and Cef.GetGlobalCookieManager and they both return null if called before Cef.Initialize
So it shouldn't be possible to actually access the cookie manager/RequestContext. Improved the xml doc

Resolves cefsharp#2600

Fix some xml doc encoding issues

Upgrade to CEF 3.3497.1841.g7f37a0a / Chromium 69.0.3497.100

Xml Doc - Add more comments and fixes

Cef.Shutdown - Improve Thread Check error message