Update GetRandomString() in DnsLatency routine

Fix the logic to append correctly.

BUG=chromium:956783
TEST=unit_tests && ./out/Default/unit_tests
--gtest_filter=DnsLatencyRoutineTest.*

Change-Id: I08d7434c2816c696c08d6fedcdd3d522ef3a7edf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2358598
Commit-Queue: Kartik Hegde <khegde@chromium.org>
Reviewed-by: Steven Bennetts <stevenjb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#799814}

chrome/browser/chromeos/net/network_diagnostics/dns_latency_routine.cc

@@ -44,7 +44,7 @@ constexpr char kHostSuffix[] = "-ccd-testing-v4.metric.gstatic.com";
 const std::string GetRandomString(int length) {
   std::string prefix;
   for (int i = 0; i < length; i++) {
-    prefix = 'a' + base::RandInt(0, 25);
+    prefix += ('a' + base::RandInt(0, 25));
   }
   return prefix;
 }