Comment that gyp_managed_install requires a userdebug flashed device.

build/android/developer_recommended_flags.gypi

 # Copyright 2013 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 # This is the set of recommended gyp variable settings for Chrome for Android development.
 #
 # These can be used by copying this file to $CHROME_SRC/chrome/supplement.gypi.
 #
 # Even better, create chrome/supplement.gypi containing the following:
 #   {
@@ skipping 20 matching lines @@
     },
     'component%': '<(component)',
 
     # When gyp_managed_install is set to 1, building an APK will install that APK on the connected
     # device(/emulator). To install on multiple devices (or onto a new device), build the APK once
     # with each device attached. This greatly reduces the time required for incremental builds.
     #
     # This comes with some caveats:
     #   Only works with a single device connected (it will print a warning if
     #     zero or multiple devices are attached).
+    #   Device must be flashed with a user-debug unsigned Android build.
     #   Some actions are always run (i.e. ninja will never say "no work to do").
     'gyp_managed_install%': 1,
 
     # With gyp_managed_install, we do not necessarily need a standalone APK.
     # When create_standalone_apk is set to 1, we will build a standalone APK
     # anyway. For even faster builds, you can set create_standalone_apk to 0.
     'create_standalone_apk%': 1,
 
     # Set clang to 1 to use the clang compiler. Clang has much (much, much) better warning/error
     # messages than gcc.
     # TODO(cjhopman): Enable this when http://crbug.com/156420 is addressed. Until then, users can
     # set clang to 1, but Android stack traces will sometimes be incomplete.
     #'clang%': 1,
 
     # Set fastbuild to 1 to build with less debugging information. This can greatly decrease linking
     # time. The downside is that stack traces will be missing useful information (like line
     # numbers).
     #'fastbuild%': 1,
   },
 }