This task covers the list of changes that we made to support building chromium with system libraries. Identify them and try to reduce their number if possible.

There are 4 categories:
 * Unbundle changes: patching the BUILD.gn files of a library (mostly adds a pkg-config and a shim-header call)
 * Fixes for building with system libs: patching .h and .cc files to fix api changes of the lib
 * Build script fixes: patching pkg-config and shim-header templates and scripts
 * qt3rdparty libs: same as Unbundle but for qt's libs

The Unbundle changes could be squashed, because we use the same pattern for every file e.g.: https://codereview.qt-project.org/c/qt/qtwebengine-chromium/+/186118 . These changes rarely need fixups.

The build fixes should have separate changes for each library. There are 3-4 libs which need these fixes frequently e.g.: https://codereview.qt-project.org/c/qt/qtwebengine-chromium/+/427771 .

The script fixes are small and probably could fit into the Unbundle patch, but some of them targeting a specific issue and and maybe better to let alone. e.g: https://codereview.qt-project.org/c/qt/qtwebengine-chromium/+/256785 and https://codereview.qt-project.org/c/qt/qtwebengine-chromium/+/529211 

The qt3rdparty libs changes could be squashed for same reason as the Unbundle changes.