Annotate NewUsbBackend feature flag with launch blocking TODO

We'll need to address https://crbug.com/1492383 before we can enable
NewUsbBackend for Mac OS. Adding this TODO by the feature flag to make
sure we don't forget to resolve the issue in the future.

Change-Id: I942f19ca730581fda24656324e12216a7fa8210c
Bug: 1492383,1096743
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/5124493
Reviewed-by: Matt Reynolds <mattreynolds@chromium.org>
Commit-Queue: Javier Castro <jacastro@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1238113}

device/base/features.cc

@@ -9,6 +9,7 @@
 namespace device {
 
 #if BUILDFLAG(IS_MAC)
+// TODO(crbug.com/1492383): Resolve or mitigate this issue before launching.
 BASE_FEATURE(kNewUsbBackend,
              "NewUsbBackend",
              base::FEATURE_DISABLED_BY_DEFAULT);