Core - LoadUrlAsync require url and remove SyncContext param

[amaitland]

Currently it's possible to call await browser.LoadUrlAsync() without passing in a Url.

If you do this and the browser isn't loading then the method will just wait forever.

To provide a predictable behaviour the string url = null will be changed to string url and an ArgumentNullException thrown when string.IsNullOrEmpty(url).

• Remove default null value for url param.
• Remove SyncronisationContext param
  upon remove it doesn't make sense
• Add WaitForInitialLoadAsync method

[amaitland]

WaitForBrowserInitialLoadAsync has been added, still needs some additional testing.

The intention of this method is to wait for the browser to finish it's initial load though it's not strictly checking for anything that prevents it from being called later.

I might consider renaming to WaitForBrowserLoadAsync before the 95 branch is created.

[amaitland]

WaitForBrowserInitialLoadAsync has been added, still needs some additional testing.

Was reverted as some of the tests failed.

[amaitland]

For anyone using LoadUrlAsync() without params then you can switch to using WaitForInitialLoadAsync for the initial page load.

//Before
await browser.LoadUrlAsync();
//After
await browser.WaitForInitialLoadAsync();