Bug 1627707 - Rename LoadInfo::LoadingPrincipal to GetLoadingPrincipal as it can return null. r=ckerschb

Mostly a matter of:

  rg -l '\->LoadingPrincipal' | xargs sed -i 's/->LoadingPrincipal/->GetLoadingPrincipal/g'

And then clang-format. But I tweaked manually nsHttpChannelAuthProvider (move
the variable where it's used, don't take a useless strong ref),
AddonContentPolicy (move the declaration of the variable to the if condition),
and BackgroundUtils (same).

Differential Revision: https://phabricator.services.mozilla.com/D69828

docshell/base/nsDocShell.cpp

@@ -11162,9 +11162,9 @@ nsresult nsDocShell::AddToSessionHistory(
     // For now keep storing just the principal in the SHEntry.
     if (!principalToInherit) {
       if (loadInfo->GetLoadingSandboxed()) {
-        if (loadInfo->LoadingPrincipal()) {
+        if (loadInfo->GetLoadingPrincipal()) {
           principalToInherit = NullPrincipal::CreateWithInheritedAttributes(
-              loadInfo->LoadingPrincipal());
+              loadInfo->GetLoadingPrincipal());
         } else {
           // get the OriginAttributes
           OriginAttributes attrs;