Add a DCHECK in omnibox_view_ios.

I suspect that processing_user_event is always true.
If this is the case, the textfield delegate can be simplified. In
order to verify this, add a DCHECK and see if it fires on the bots.

Cq-Include-Trybots: luci.chromium.try:ios-simulator-full-configs;master.tryserver.chromium.mac:ios-simulator-cronet
Change-Id: Ic45526950bf1f4b05a4c11a9cde551d0eef6dc0e
Reviewed-on: https://chromium-review.googlesource.com/1120816Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Stepan Khapugin <stkhapugin@chromium.org>
Cr-Commit-Position: refs/heads/master@{#573710}

ios/chrome/browser/ui/omnibox/omnibox_view_ios.mm

@@ -545,7 +545,7 @@ bool OmniboxViewIOS::OnWillChange(NSRange range, NSString* new_text) {
 
 void OmniboxViewIOS::OnDidChange(bool processing_user_event) {
   omnibox_interacted_while_focused_ = YES;
-
+  DCHECK(processing_user_event);
   // Sanitize pasted text.
   if (model()->is_pasting()) {
     base::string16 pastedText = base::SysNSStringToUTF16([field_ text]);