I believe this would apply to clank as well as WebView, but this certainly applies to WebView.

Commandline flags are only read from the flag file on debuggable devices (userdebug, eng). Right now this isn't super obvious when using commandline-setting scripts.

Wishlist (all of the following assume a user device):

 * `build/android/adb_system_webview_command_line` (with no args) should print something like "flags are not supported on user devices"
 * `build/android/adb_system_webview_command_line --some-flag` should fail with an explicit error message stating that flags are not supported (right now it fails, but it's not obvious why)
 * ^ all of the above, but for adb_chrome_public_command_line, Generated Wrapper Scripts, etc.

Despite bullet point number 2, it's still actually possible to write to the flags file manually. In this case, per bullet 1, the scripts print the value of the file (even though such a value is not respected by the product). So, I think both bullet points are justified.