Qt WebEngine: Force treating source code as UTF-8 on Windows

A customer reported this:

Since my Windows is Korean language, MSVC tries to decode all source code in CP949 but some source codes from gn and chromium contains non-ASCII characters and they are obviously encoded in UTF-8. This can be easily fixed by adding '/utf-8' flag when compiling.

Would it make sense to set CMAKE_CXX_FLAGS="/utf-8" and CMAKE_C_FLAGS="/utf-8" automatically for MSVC builds? (see https://stackoverflow.com/questions/47690822/possible-to-force-cmake-msvc-to-use-utf-8-encoding-for-source-files-without-a-bo )

Also, https://github.com/qt/qtwebengine-chromium/blob/772cb04f511b9a1b98a4e155ed62efdfd971e5fb/chromium/tools/idl_parser/idl_parser.py#L1318 raises an invalid byte sequence error when the wrong encoding is assumed. (To be patched upstream?)

- with open(filename) as fileobject:
+ with open(filename, encoding="utf-8") as fileobject: