Restrict availability of libusb to macOS and Windows

On all other platforms //device/usb uses a custom backend. This patch
ensures that we are not including libusb in unexpected build
configurations.

Change-Id: Ia1b68e86048af7ed98d27b19f46f74ceba50097b
Reviewed-on: https://chromium-review.googlesource.com/996407Reviewed-by: Jun Cai <juncai@chromium.org>
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#548282}

chrome/browser/chromeos/BUILD.gn

@@ -186,7 +186,6 @@ source_set("chromeos") {
     "//third_party/cacheinvalidation",
     "//third_party/icu",
     "//third_party/leveldatabase",
-    "//third_party/libusb",
     "//third_party/protobuf:protobuf_lite",
     "//third_party/protobuf:protoc($host_toolchain)",
     "//third_party/re2",

third_party/libusb/BUILD.gn

@@ -2,7 +2,8 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-assert(!is_android && !is_ios)
+# libusb is only used by //device/usb on Windows and macOS.
+assert(is_win || is_mac)
 
 import("//build/config/features.gni")
 
@@ -22,6 +23,10 @@ config("libusb_warnings") {
 }
 
 static_library("libusb") {
+  visibility = [
+    "//device:device_unittests",
+    "//device/usb",
+  ]
   sources = [
     "src/config.h",
     "src/libusb/core.c",