Bug 1594529: Infer nsDocShell::IsFrame from BrowsingContext. r=nika

This fixes the content policy type for document loads in these frames, where
the explicit mIsFrame flag was not set, due to DocShell creation taking a
different code path in remote frames than in in-process frames.

Differential Revision: https://phabricator.services.mozilla.com/D52093

docshell/base/nsDocShell.cpp

@@ -388,7 +388,6 @@ nsDocShell::nsDocShell(BrowsingContext* aBrowsingContext,
       mHasLoadedNonBlankURI(false),
       mBlankTiming(false),
       mTitleValidForCurrentURI(false),
-      mIsFrame(false),
       mSkipBrowsingContextDetachOnDestroy(false),
       mWatchedByDevtools(false),
       mIsNavigating(false) {
@@ -12088,8 +12087,6 @@ nsresult nsDocShell::EnsureFind() {
   return NS_OK;
 }
 
-bool nsDocShell::IsFrame() { return mIsFrame; }
-
 NS_IMETHODIMP
 nsDocShell::IsBeingDestroyed(bool* aDoomed) {
   NS_ENSURE_ARG(aDoomed);