Prerender: Check SpeculationHostDelegate is non-null before using

This CL fixes the issue that SpeculationHostImpl should ensure the
delegate pointer is not null before using it.

Bug: 1197133
Change-Id: I3a37f86983307571fe6368ff859b481ec518cabb
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2907013
Reviewed-by: Tsuyoshi Horo <horo@chromium.org>
Commit-Queue: Lingqi Chi <lingqi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#884858}

content/browser/speculation_rules/speculation_host_impl.cc

@@ -55,7 +55,8 @@ void SpeculationHostImpl::UpdateSpeculationCandidates(
     return;
 
   // Let `delegate_` process the candidates that it is interested in.
-  delegate_->ProcessCandidates(candidates);
+  if (delegate_)
+    delegate_->ProcessCandidates(candidates);
 
   // TODO(crbug.com/1197133): process prerender candidates.
 }