Decouple the QT6 and QT5 dependencies to support building against one without the other.  

Taken from: https://issues.chromium.org/issues/328182252#comment3

--- a/chrome/installer/linux/BUILD.gn
+++ b/chrome/installer/linux/BUILD.gn
@@ -81,10 +81,12 @@
   # Even though this is a shared library, add it to `packaging_files` instead of
   # `packaging_files_shlibs` to skip the dependency check.  This is intentional
   # to prevent a hard dependency on QT for the package.
-  packaging_files += [ "$root_out_dir/libqt5_shim.so" ]
-}
-if (use_qt6) {
-  packaging_files += [ "$root_out_dir/libqt6_shim.so" ]
+  if (use_qt5 {
+    packaging_files += [ "$root_out_dir/libqt5_shim.so" ]
+  }
+  if (use_qt6) {
+    packaging_files += [ "$root_out_dir/libqt6_shim.so" ]
+  }
 }
 
 action_foreach("calculate_deb_dependencies") {
@@ -209,15 +211,17 @@
 }
 
 if (use_qt) {
-  strip_binary("strip_qt5_shim") {
-    binary_input = "$root_out_dir/libqt5_shim.so"
-    deps = [ "//ui/qt:qt5_shim" ]
+  if (use_qt5) {
+    strip_binary("strip_qt5_shim") {
+      binary_input = "$root_out_dir/libqt5_shim.so"
+      deps = [ "//ui/qt:qt5_shim" ]
+    }
   }
-}
-if (use_qt6) {
-  strip_binary("strip_qt6_shim") {
-    binary_input = "$root_out_dir/libqt6_shim.so"
-    deps = [ "//ui/qt:qt6_shim" ]
+  if (use_qt6) {
+    strip_binary("strip_qt6_shim") {
+      binary_input = "$root_out_dir/libqt6_shim.so"
+      deps = [ "//ui/qt:qt6_shim" ]
+    }
   }
 }
 
@@ -401,16 +405,18 @@
     ]
   }
   if (use_qt) {
-    public_deps += [
-      ":strip_qt5_shim",
-      "//ui/qt:qt5_shim",
-    ]
-  }
-  if (use_qt6) {
-    public_deps += [
-      ":strip_qt6_shim",
-      "//ui/qt:qt6_shim",
-    ]
+    if (use_qt5) {
+      public_deps += [
+        ":strip_qt5_shim",
+        "//ui/qt:qt5_shim",
+      ]
+    }
+    if (use_qt6) {
+      public_deps += [
+        ":strip_qt6_shim",
+        "//ui/qt:qt6_shim",
+      ]
+    }
   }
 }
 
--- a/ui/qt/BUILD.gn
+++ b/ui/qt/BUILD.gn
@@ -61,6 +61,7 @@
     packages = [
       "Qt" + invoker.qt_version + "Core",
       "Qt" + invoker.qt_version + "Widgets",
+      "Qt" + invoker.qt_version + "Gui",
     ]
   }
 
@@ -101,10 +101,12 @@
     }
   }
 }
-qt_shim("qt5_shim") {
-  qt_version = "5"
-  if (!use_sysroot) {
-    moc_qt_path = "$moc_qt5_path"
+if (use_qt5) {
+  qt_shim("qt5_shim") {
+    qt_version = "5"
+    if (!use_sysroot) {
+      moc_qt_path = "$moc_qt5_path"
+    }
   }
 }
 if (use_qt6) {
@@ -122,7 +124,10 @@
   defines = [ "IS_QT_IMPL" ]
 
+  data_deps = []
   # qt_shim is in data_deps since we want to load it manually.
-  data_deps = [ ":qt5_shim" ]
+  if (use_qt5) {
+    data_deps += [ ":qt5_shim" ]
+  }
   if (use_qt6) {
     data_deps += [ ":qt6_shim" ]
   }
--- a/ui/qt/qt.gni
+++ b/ui/qt/qt.gni
@@ -13,7 +13,11 @@
 }
 
 declare_args() {
-  if (!use_sysroot && use_qt) {
+  use_qt5 = use_qt && use_sysroot
+}
+
+declare_args() {
+  if (!use_sysroot && use_qt5) {
     moc_qt5_path = ""
   }
 }
@@ -29,4 +33,5 @@
 }
 
 # use_qt6 => use_qt
+assert(!use_qt5 || use_qt)
 assert(!use_qt6 || use_qt)