Dispatch UsbChooserContext::GetDevices callback asynchronously

In r604906 this method was updated to return the device list from cache
if available. By running the passed callback synchronously however this
violated the assumptions of other code, such as the USB device chooser
on Android.

Methods that execute a callback either synchronously or asynchronously
tend to trigger errors in other code and so rather than work around this
change in behavior this patch forces the callback to always be run
asynchronously.

Bug: 904321
Change-Id: Ib6a71bee3c578e42e2812bc762db40ebc2bd6994
Reviewed-on: https://chromium-review.googlesource.com/c/1338941
Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Ken Rockot <rockot@google.com>
Cr-Commit-Position: refs/heads/master@{#608912}

chrome/browser/usb/usb_chooser_context.cc

@@ -10,6 +10,7 @@
 #include "base/metrics/histogram_macros.h"
 #include "base/stl_util.h"
 #include "base/strings/utf_string_conversions.h"
+#include "base/threading/sequenced_task_runner_handle.h"
 #include "base/values.h"
 #include "chrome/browser/profiles/profile.h"
 #include "chrome/browser/usb/usb_blocklist.h"
@@ -279,15 +280,16 @@ bool UsbChooserContext::HasDevicePermission(
 
 void UsbChooserContext::GetDevices(
     device::mojom::UsbDeviceManager::GetDevicesCallback callback) {
-  if (is_initialized_) {
-    std::vector<device::mojom::UsbDeviceInfoPtr> device_list;
-    for (const auto& pair : devices_) {
-      device_list.push_back(pair.second->Clone());
-    }
-    std::move(callback).Run(std::move(device_list));
-  } else {
+  if (!is_initialized_) {
     pending_get_devices_requests_.push(std::move(callback));
+    return;
   }
+
+  std::vector<device::mojom::UsbDeviceInfoPtr> device_list;
+  for (const auto& pair : devices_)
+    device_list.push_back(pair.second->Clone());
+  base::SequencedTaskRunnerHandle::Get()->PostTask(
+      FROM_HERE, base::BindOnce(std::move(callback), std::move(device_list)));
 }
 
 void UsbChooserContext::GetDevice(