Settings hardening: add a test for empty legacy device ids

The test validates that empty legacy device ids properly validate prefs
as VALID_SECURE_LEGACY. The name and expectations will be changed in
https://chromium-review.googlesource.com/c/chromium/src/+/655929 which
disallows empty legacy device ids.

Bug: 500085
Change-Id: Id99434c6441928b0faaeb1d5f7357d99fe8ef9a6
Reviewed-on: https://chromium-review.googlesource.com/685483
Commit-Queue: Gabriel Charette <gab@chromium.org>
Reviewed-by: Gabriel Charette <gab@chromium.org>
Cr-Commit-Position: refs/heads/master@{#504480}

services/preferences/tracked/pref_hash_calculator_unittest.cc

@@ -196,3 +196,17 @@ TEST(PrefHashCalculatorTest, TestCompatibilityWithLegacyDeviceId) {
             PrefHashCalculator(kSeed, kNewDeviceId, kLegacyDeviceId)
                 .Validate("pref.path", &string_value, kExpectedValue));
 }
+
+TEST(PrefHashCalculatorTest, TestCompatibleWithEmptyLegacyDeviceId) {
+  static const char kSeed[] = "0123456789ABCDEF0123456789ABCDEF";
+  static const char kNewDeviceId[] = "unused";
+  static const char kLegacyDeviceId[] = "";
+
+  const base::Value string_value("testing with special chars:\n<>{}:^^@#$\\/");
+  static const char kExpectedValue[] =
+      "F14F989B7CAABF3B36ECAE34492C4D8094D2500E7A86D9A3203E54B274C27CB5";
+
+  EXPECT_EQ(PrefHashCalculator::VALID_SECURE_LEGACY,
+            PrefHashCalculator(kSeed, kNewDeviceId, kLegacyDeviceId)
+                .Validate("pref.path", &string_value, kExpectedValue));
+}