Mark RenderFrameHostImpl.parent_ as const.

I added a helper GetParentRenderFrameHost because the previous code
would crash if current_render_frame_host() was nullptr. This ensures
that we still crash if that ever happens.

Change-Id: I0f9d3ef2f5cb1a86a247f5fe5db8b6d9fe47d31a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2041153Reviewed-by: Arthur Sonzogni <arthursonzogni@chromium.org>
Commit-Queue: Fergal Daly <fergal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740616}

content/browser/frame_host/render_frame_host_impl.cc

@@ -880,7 +880,10 @@ RenderFrameHostImpl::RenderFrameHostImpl(
       process_(site_instance->GetProcess()),
       frame_tree_(frame_tree),
       frame_tree_node_(frame_tree_node),
-      parent_(nullptr),
+      // If it has a parent, that parent must have a RFH.
+      parent_(frame_tree_node_->parent()
+                  ? frame_tree_node_->parent()->current_frame_host()
+                  : nullptr),
       routing_id_(routing_id),
       is_waiting_for_unload_ack_(false),
       render_frame_created_(false),
@@ -924,11 +927,6 @@ RenderFrameHostImpl::RenderFrameHostImpl(
   GetSiteInstance()->IncrementActiveFrameCount();
 
   if (frame_tree_node_->parent()) {
-    // Keep track of the parent RenderFrameHost, which shouldn't change even if
-    // this RenderFrameHost is on the pending deletion list and the parent
-    // FrameTreeNode has changed its current RenderFrameHost.
-    parent_ = frame_tree_node_->parent()->current_frame_host();
-
     cross_origin_embedder_policy_ = parent_->cross_origin_embedder_policy();
 
     // New child frames should inherit the nav_entry_id of their parent.

content/browser/frame_host/render_frame_host_impl.h

@@ -2110,8 +2110,10 @@ class CONTENT_EXPORT RenderFrameHostImpl
   // The active parent RenderFrameHost for this frame, if it is a subframe.
   // Null for the main frame.  This is cached because the parent FrameTreeNode
   // may change its current RenderFrameHost while this child is pending
-  // deletion, and GetParent() should never return a different value.
-  RenderFrameHostImpl* parent_;
+  // deletion, and GetParent() should never return a different value, even if
+  // this RenderFrameHost is on the pending deletion list and the parent
+  // FrameTreeNode has changed its current RenderFrameHost.
+  RenderFrameHostImpl* const parent_;
 
   // Track this frame's last committed URL.
   GURL last_committed_url_;