Fix compile error emitted by trunk clang

Error introduced by https://github.com/llvm/llvm-project/commit/9ce6dc98
    on implicit nullptr to bool conversions that clang used to warn on.

Bug: 1051175
Change-Id: If13318abe4fcc6336104bf730e6671d82f1834af
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2050034
Commit-Queue: Amy Huang <akhuang@google.com>
Reviewed-by: Eric Orth <ericorth@chromium.org>
Cr-Commit-Position: refs/heads/master@{#740480}

net/dns/dns_transaction_unittest.cc

@@ -834,7 +834,7 @@ class DnsTransactionTestBase : public testing::Test {
 
     request_context_ = std::make_unique<TestURLRequestContext>();
     resolve_context_ = std::make_unique<ResolveContext>(
-        request_context_.get(), nullptr /* host_cache */);
+        request_context_.get(), false /* enable_caching */);
 
     ConfigureFactory();
   }