Build CupsProxyService cups_utils tests under use_cups

Didn't realize this test suite became dependent on cups at some point;
this change enforces this.

Bug: chromium:945409, chromium:988567
Test: unittests built successfully
Change-Id: I109fb2c667daf6cf7178c4cd1e67ef8f4c40658d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1764633Reviewed-by: Sean Kau <skau@chromium.org>
Commit-Queue: Luum Habtemariam <luum@chromium.org>
Cr-Commit-Position: refs/heads/master@{#689247}

chrome/services/cups_proxy/public/cpp/BUILD.gn

@@ -35,13 +35,15 @@ source_set("cpp") {
 source_set("unit_tests") {
   testonly = true
 
-  sources = [
-    "cups_util_unittest.cc",
-  ]
-  deps = [
-    ":cpp",
-    "//base",
-    "//testing/gmock",
-    "//testing/gtest",
-  ]
+  if (use_cups) {
+    sources = [
+      "cups_util_unittest.cc",
+    ]
+    deps = [
+      ":cpp",
+      "//base",
+      "//testing/gmock",
+      "//testing/gtest",
+    ]
+  }
 }