CrOS: Fix networking errors in informational builders.

Tast.NetworkEventLog is showing some frequent errors. This demotes or
modifies the errors to avoid spam on frequently loaded builders.
https://tastboard.corp.google.com/test/logs.NetworkEventLog

Bug: b/167581118
Change-Id: I23520471f9fc14fc7a4200143c9ac7e557cd37b1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2430024Reviewed-by: Azeem Arshad <azeemarshad@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Commit-Queue: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#810384}

chrome/browser/chromeos/net/network_portal_detector_impl.cc

@@ -377,7 +377,7 @@ void NetworkPortalDetectorImpl::OnAttemptTimeout() {
   DCHECK_CALLED_ON_VALID_SEQUENCE(sequence_checker_);
   DCHECK(is_checking_for_portal());
 
-  NET_LOG(ERROR) << "Portal detection timeout: "
+  NET_LOG(EVENT) << "Portal detection timeout: "
                  << " id=" << NetworkGuidId(default_network_id_);
 
   captive_portal_detector_->Cancel();

chromeos/network/onc/onc_utils.cc

@@ -646,6 +646,11 @@ const char kEmptyUnencryptedConfiguration[] =
     "\"Certificates\":[]}";
 
 std::unique_ptr<base::Value> ReadDictionaryFromJson(const std::string& json) {
+  if (json.empty()) {
+    // Policy may contain empty values, just log a debug message.
+    NET_LOG(DEBUG) << "Empty json string";
+    return nullptr;
+  }
   base::JSONReader::ValueWithError parsed_json =
       base::JSONReader::ReadAndReturnValueWithError(
           json, base::JSON_ALLOW_TRAILING_COMMAS);

components/device_event_log/device_event_log.cc

@@ -17,7 +17,9 @@ namespace {
 const size_t kDefaultMaxEntries = 4000;
 
 const int kSlowMethodThresholdMs = 10;
-const int kVerySlowMethodThresholdMs = 50;
+// Loaded builders may perform badly, set this to a fairly high value to catch
+// extreme cases.
+const int kVerySlowMethodThresholdMs = 250;
 
 DeviceEventLogImpl* g_device_event_log = nullptr;