Merge M-45: Stablize 'usb_printers' extension manifest key.

The new permissions flow for USB printers feature is trying to launch in
M-45 and I forgot to stablize this before the branch point.

BUG=484546
[email protected],[email protected]

Review URL: https://codereview.chromium.org/1269083004

Cr-Commit-Position: refs/heads/master@{#341628}
(cherry-picked from commit 28fd987)

Review URL: https://codereview.chromium.org/1265353002 .

Cr-Commit-Position: refs/branch-heads/2454@{#227}
Cr-Branched-From: 12bfc33-refs/heads/master@{#338390}

chrome/browser/ui/webui/print_preview/extension_printer_handler_unittest.cc

@@ -20,7 +20,6 @@
 #include "chrome/browser/extensions/test_extension_environment.h"
 #include "chrome/browser/local_discovery/pwg_raster_converter.h"
 #include "chrome/browser/ui/webui/print_preview/extension_printer_handler.h"
-#include "chrome/common/extensions/features/feature_channel.h"
 #include "chrome/test/base/testing_profile.h"
 #include "device/core/device_client.h"
 #include "device/usb/mock_usb_device.h"
@@ -553,8 +552,6 @@ TEST_F(ExtensionPrinterHandlerTest, GetPrinters_Reset) {
 }
 
 TEST_F(ExtensionPrinterHandlerTest, GetUsbPrinters) {
-  extensions::ScopedCurrentChannel channel(chrome::VersionInfo::CHANNEL_DEV);
-
   scoped_refptr<MockUsbDevice> device0 =
       new MockUsbDevice(0, 0, "Google", "USB Printer", "");
   usb_service_.AddDevice(device0);

extensions/common/api/_manifest_features.json

@@ -271,7 +271,7 @@
     "extension_types": ["platform_app"]
   },
   "usb_printers": {
-    "channel": "dev",
+    "channel": "stable",
     "extension_types": ["platform_app"]
   },
   "version": {