Add parentheses to appease -Wstring-concatenation

Bug: 1114873
Change-Id: If38541941c0cf90f9947642d208bd16bd820a719
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2357515Reviewed-by: Eric Roman <eroman@chromium.org>
Reviewed-by: Will Harris <wfh@chromium.org>
Commit-Queue: Arthur Eubanks <aeubanks@google.com>
Cr-Commit-Position: refs/heads/master@{#798366}

services/network/public/cpp/proxy_config_mojom_traits_unittest.cc

@@ -62,8 +62,8 @@ TEST(ProxyConfigTraitsTest, ProxyRules) {
       "ftp=ftp1,ftp2,ftp3",
       "http=http1,http2; http=http3",
       "ftp=ftp1,ftp2,ftp3 ; http=http1,http2; ",
-      "http=https://secure_proxy; ftp=socks4://socks_proxy; "
-      "https=socks://foo",
+      ("http=https://secure_proxy; ftp=socks4://socks_proxy; "
+       "https=socks://foo"),
       "socks=foopy",
       "http=httpproxy ; https=httpsproxy ; ftp=ftpproxy ; socks=foopy ",
       "http=httpproxy ; https=httpsproxy ; socks=socks5://foopy ",