Fix LauncherSearchProvider DCHECK.

The LSP can DCHECK when calculating query/title score matches, because
the query or title text can be empty. This is partly because the LSP
runs even on zero-state searches, and (perhaps) partly because files can
have no title.

This CL adds various checks to prevent calculating scores when we don't
have both a query and title. It also stops the LSP from running at all
on zero-state searches.

Bug: 1131358
Change-Id: I65c264def66e83f1c7fd47956eb0a9ddf59e099e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2426088Reviewed-by: Thanh Nguyen <thanhdng@chromium.org>
Commit-Queue: Tony Yeoman <tby@chromium.org>
Cr-Commit-Position: refs/heads/master@{#811057}

chrome/browser/ui/app_list/search/launcher_search/launcher_search_provider.cc

@@ -38,6 +38,10 @@ constexpr double kPartialMatchPenaltyRate = 0.9;
 
 double FuzzyMatchRelevance(const TokenizedString& title,
                            const TokenizedString& query) {
+  if (title.text().empty() || query.text().empty()) {
+    return kDefaultRelevance;
+  }
+
   FuzzyTokenizedStringMatch match;
   match.IsRelevant(query, title, kRelevanceThreshold, kUsePrefixOnly,
                    kUseWeightedRatio, kUseEditDistance,
@@ -74,6 +78,10 @@ void LauncherSearchProvider::Start(const base::string16& query) {
   // Clear previously added search results.
   ClearResults();
 
+  // LauncherSearchProvider only handles query searches.
+  if (query.empty())
+    return;
+
   last_tokenized_query_.emplace(query, TokenizedString::Mode::kWords);
   DelayQuery(base::Bind(&LauncherSearchProvider::StartInternal,
                         weak_ptr_factory_.GetWeakPtr(), query));