webview: Increase fixed size by 100×100px

At the previous fixed size of 400×500px, the "Next" button on the first page of the Google login flow is not visible. It also feels, subjectively, a bit cramped.

Increasing the fixed size by 100px in each axis makes the "Next" button visible, is smaller than the default size of Initial Setup and Settings (the parent application for the dialog this view is embedded in), and also falls within the HIG-specified smallest display size of 1024×600. (The existing natural_size constants already exceed the HIG-specified minimum phone size of 360×294.)

https://developer.gnome.org/hig/guidelines/adaptive.html?highlight=size#small-size-handling

Before:

After (though I must confess I mocked this up by messing with the size request in GTK Inspector):