geolocation: fix an bugprone-string-integer-assignment instance

mac_address is a string16, so this assignment previously did
`mac_address = (char16)3;`. That seems unintended.

Bug: 1130867
Change-Id: I6cb4a5f86e1dfa0bc8eef68f7b81412006e6638f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426246
Commit-Queue: George Burgess <gbiv@chromium.org>
Reviewed-by: Matt Reynolds <mattreynolds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811797}

services/device/geolocation/wifi_data_provider_common_unittest.cc

@@ -203,7 +203,7 @@ TEST_F(GeolocationWifiDataProviderCommonTest, DoScanWithResults) {
   EXPECT_CALL(*polling_policy_, PollingInterval()).Times(AtLeast(1));
   AccessPointData single_access_point;
   single_access_point.channel = 2;
-  single_access_point.mac_address = 3;
+  single_access_point.mac_address = base::ASCIIToUTF16("00:11:22:33:44:55");
   single_access_point.radio_signal_strength = 4;
   single_access_point.signal_to_noise = 5;
   single_access_point.ssid = base::ASCIIToUTF16("foossid");