Don't try to create a verbatim match with an empty input.

components/omnibox/browser/clipboard_url_provider.cc

 // Copyright 2015 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.
 
 #include "components/omnibox/browser/clipboard_url_provider.h"
 
 #include "base/logging.h"
 #include "base/strings/utf_string_conversions.h"
 #include "components/omnibox/browser/autocomplete_input.h"
 #include "components/omnibox/browser/autocomplete_provider_client.h"
@@ skipping 12 matching lines @@
       clipboard_content_(clipboard_content),
       history_url_provider_(history_url_provider) {
   DCHECK(clipboard_content_);
 }
 
 ClipboardURLProvider::~ClipboardURLProvider() {}
 
 void ClipboardURLProvider::Start(const AutocompleteInput& input,
                                  bool minimal_changes) {
   matches_.clear();
+
+  // If the user started typing, do not offer clipboard based match.

[Peter Kasting, 2016/07/18 22:16:15]

Nit: I would not add these next two comments.  They simply restate the code
without adding any useful information such as motivation why we do this, so
they're just clutter.

   if (!input.from_omnibox_focus())
     return;
 
   GURL url;
+  // If the clipboard does not contain any URL, or the URL on the page is the
+  // same as the URL in the clipboard, early return.
   if (!clipboard_content_->GetRecentURLFromClipboard(&url) ||
       url == input.current_url())
     return;
 
   DCHECK(url.is_valid());
-  // Adds a default match. This match will be opened when the user presses "Go".
-  AutocompleteMatch verbatim_match = VerbatimMatchForURL(
-      client_, input, input.current_url(), history_url_provider_, -1);
-  if (verbatim_match.destination_url.is_valid())
+  // If the omnibox is not empty, add a default match.

[Peter Kasting, 2016/07/18 22:16:15]

Again, this adds detail that just restates the code.  If you think the old
comment is lacking, focus on describing why it matters that the omnibox is
non-empty, instead of just restating the code.

+  // This match will be opened when the user presses "Enter".
+  if (!input.text().empty()) {
+    AutocompleteMatch verbatim_match = VerbatimMatchForURL(
+        client_, input, input.current_url(), history_url_provider_, -1);
     matches_.push_back(verbatim_match);
+  }
 
-  // Add a clipboard match just below the verbatim match.
-  AutocompleteMatch match(this, verbatim_match.relevance - 1, false,
-                          AutocompleteMatchType::CLIPBOARD);
+  // Add the clipboard match. The relevance is 800 to beat ZeroSuggest results.
+  AutocompleteMatch match(this, 800, false, AutocompleteMatchType::CLIPBOARD);

[Peter Kasting, 2016/07/18 22:16:15]

I don't think we should hardcode 800 here.  If there is a verbatim match, using
its relevance - 1, as before, seems correct.  If there isn't, will there ever be
ZeroSuggest results either?  If not, then something like max(1, verbatim
relevance - 1) would probably be correct.

   match.destination_url = url;
   match.contents.assign(url_formatter::FormatUrl(
       url, url_formatter::kFormatUrlOmitAll, net::UnescapeRule::SPACES,
       nullptr, nullptr, nullptr));
   AutocompleteMatch::ClassifyLocationInString(
       base::string16::npos, 0, match.contents.length(),
       ACMatchClassification::URL, &match.contents_class);
 
   match.description.assign(l10n_util::GetStringUTF16(IDS_LINK_FROM_CLIPBOARD));
   AutocompleteMatch::ClassifyLocationInString(
       base::string16::npos, 0, match.description.length(),
       ACMatchClassification::NONE, &match.description_class);
 
   // At least one match must be default, so if verbatim_match was invalid,
   // the clipboard match is allowed to be default.
   match.allowed_to_be_default_match = matches_.empty();
   matches_.push_back(match);
 }