Hide Arc USB host features behind flag.

Hide Arc USB host features behind a flag. Android side always gets empty
device list when the flag is disabled.

Bug: 776476
Test: Manual
Change-Id: I0e6aeea60e241dc9c5b4ae1f035cfe2fd064ca9d
Reviewed-on: https://chromium-review.googlesource.com/854990
Commit-Queue: Long Cheng <[email protected]>
Reviewed-by: Luis Hector Chavez <[email protected]>
Cr-Commit-Position: refs/heads/master@{#527893}
diff --git a/chrome/browser/flag_descriptions.h b/chrome/browser/flag_descriptions.h
index e30487c1..d2bf2e5 100644
--- a/chrome/browser/flag_descriptions.h
+++ b/chrome/browser/flag_descriptions.h
@@ -1497,6 +1497,9 @@
 extern const char kArcNativeBridgeExperimentName[];
 extern const char kArcNativeBridgeExperimentDescription[];
 
+extern const char kArcUsbHostName[];
+extern const char kArcUsbHostDescription[];
+
 extern const char kArcVpnName[];
 extern const char kArcVpnDescription[];