[bug#46337] Clarification for the previous patch

[Sergey Trofimov]

Hey there. Just to clarify why and how this patch works:
When `-webengine-proprietary-codecs` flag is added `rtc_use_h264` 
gn arg would be enabled. [1]
This flag under the hood adds dependency on openh264 (to encode 
webrtc streams). [2]

Now to make the build accept system library we have to jump 
through some hoops:
- List of config flags [3] which qtwebengine supports misses 
 webengine-system-openh264
- Chromium on the other hand supports linking with system library. 
 [4]
- To pass the required GN arg I blatantly slap it in linux.pri to 
 gn_args

[1] 
https://github.com/qt/qtwebengine/blob/v5.15.2/src/core/config/common.pri#L31
[2] 
https://github.com/qt/qtwebengine-chromium/blob/88-based/chromium/third_party/webrtc/modules/video_coding/BUILD.gn#L402
[3] 
https://github.com/qt/qtwebengine/blob/v5.15.2/src/buildtools/configure.json#L763
[4] 
https://github.com/qt/qtwebengine-chromium/blob/88-based/chromium/third_party/openh264/openh264_args.gni