I think you can’t just “compile” Webkit for WinRT / Metro Style. Metro Style applications are limited in the form of API calls that they can make, for example, for WinRT there is no GDI / GDI + / MFC. WebKit has several build modes that you can use, you can build it using QT as a rendering engine or using GTK or a simple GDI, but in all these cases, when creating new builds for Windows, you will use GDI at the very end.
However, you can change the source code of Webkit and add a new rendering engine that uses the new WinRT APIs. You could be famous if you do.
As a side note, even when there is a Chrome version for the Windows Store, looking at the source code of Chromium, it seems to me that this version is just a simple application launcher that communicates with the regular desktop version using the IPC channel. It is not a real build of the Windows Store of all source code.
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