CodeHealth: Remove use of Value::GetAsString.

This change removes the use of Value::GetAsString and replaces with
Value::is_string, Value::GetString(), Value::GetIfString().

This CL was uploaded by git cl split.

R=rsleevi@chromium.org

Bug: 1187007
Change-Id: I25acdcb3c0ca2d232d6cffbff731ad6731ea2b27
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3098783
Auto-Submit: Sammie Quon <sammiequon@chromium.org>
Reviewed-by: Ryan Sleevi <rsleevi@chromium.org>
Commit-Queue: Ryan Sleevi <rsleevi@chromium.org>
Cr-Commit-Position: refs/heads/main@{#913472}

net/cert/ct_log_response_parser.cc

@@ -112,11 +112,11 @@ struct JsonConsistencyProof {
 };
 
 bool ConvertIndividualProofNode(const base::Value* value, std::string* result) {
-  std::string b64_encoded_node;
-  if (!value->GetAsString(&b64_encoded_node))
+  const std::string* b64_encoded_node = value->GetIfString();
+  if (!b64_encoded_node)
     return false;
 
-  if (!ConvertSHA256RootHash(b64_encoded_node, result))
+  if (!ConvertSHA256RootHash(*b64_encoded_node, result))
     return false;
 
   return true;