Migrate to new StatisticsProvider API.

Use new getters introduced in crrev.com/c/3963072. The new getters
return optional string piece instead of returning bool and copying
result into string provided as a pointer argument.

Handling optional in callers code makes caller explicitly handle missing
statistic.

Usage of base::StringPiece reduces number of copies for both keys and
returned statistics. Callers have to make an explicit copy if needed.

This CL was uploaded by git cl split.

R=sdefresne@chromium.org

Bug: b:213325251
Test: unit_tests
Test: browser_tests
Change-Id: I731baea102e9d574743500808a63f9317283903a
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4031029
Commit-Queue: Sylvain Defresne <sdefresne@chromium.org>
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Auto-Submit: Artem Sumaneev <asumaneev@google.com>
Cr-Commit-Position: refs/heads/main@{#1072241}

rlz/chromeos/lib/rlz_value_store_chromeos.cc

@@ -379,9 +379,9 @@ bool RlzValueStoreChromeOS::IsStatefulEvent(Product product,
   if (strcmp(event_rlz, "CAF") == 0) {
     chromeos::system::StatisticsProvider* stats =
         chromeos::system::StatisticsProvider::GetInstance();
-    std::string should_send_rlz_ping_value;
+    if (const absl::optional<base::StringPiece> should_send_rlz_ping_value =
-    if (stats->GetMachineStatistic(chromeos::system::kShouldSendRlzPingKey,
+            stats->GetMachineStatistic(
-                                   &should_send_rlz_ping_value)) {
+                chromeos::system::kShouldSendRlzPingKey)) {
       if (should_send_rlz_ping_value ==
           chromeos::system::kShouldSendRlzPingValueFalse) {
         return true;
@@ -389,7 +389,7 @@ bool RlzValueStoreChromeOS::IsStatefulEvent(Product product,
                  chromeos::system::kShouldSendRlzPingValueTrue) {
         LOG(WARNING) << chromeos::system::kShouldSendRlzPingKey
                      << " has an unexpected value: "
-                     << should_send_rlz_ping_value << ". Treat it as "
+                     << should_send_rlz_ping_value.value() << ". Treat it as "
                      << chromeos::system::kShouldSendRlzPingValueFalse
                      << " to avoid sending duplicate rlz ping.";
         return true;