Remove unused parts of IPC::ChannelHandle.

android_webview/browser/aw_printing_message_filter.h

Index: android_webview/browser/aw_printing_message_filter.h
diff --git a/android_webview/browser/aw_printing_message_filter.h b/android_webview/browser/aw_printing_message_filter.h
index a16eddd6fd5f64eae8df8c95f36b149499ea0657..b4a20e24b2358d5f3f9e89361af4f9ff8c924c68 100644
--- a/android_webview/browser/aw_printing_message_filter.h
+++ b/android_webview/browser/aw_printing_message_filter.h
@@ -8,6 +8,10 @@
 #include "base/macros.h"
 #include "content/public/browser/browser_message_filter.h"
 
+namespace base {
+struct FileDescriptor;
+}
+
 namespace android_webview {
 
 // This class filters out incoming printing related IPC messages for the