[fuzzy-app-search] Fix bug regarding EditDistance ratio

If the edit distance is big enough, the ratio could be less than 0.
This CL fixes the bug.

Bug: 990684
Change-Id: I61e536aff49cef3cd5e844b816f8d3e6662f0e88
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2068228Reviewed-by: Rachel Wong <wrong@chromium.org>
Commit-Queue: Thanh Nguyen <thanhdng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#743878}

chrome/common/string_matching/sequence_matcher.cc

@@ -16,7 +16,7 @@ bool CompareMatches(const Match& m1, const Match& m2) {
 }
 }  // namespace
 
-SequenceMatcher::Match::Match() {}
+SequenceMatcher::Match::Match() = default;
 SequenceMatcher::Match::Match(int pos_first, int pos_second, int len)
     : pos_first_string(pos_first), pos_second_string(pos_second), length(len) {
   DCHECK_GE(pos_first_string, 0);
@@ -175,9 +175,9 @@ double SequenceMatcher::Ratio() {
   if (use_edit_distance_) {
     if (edit_distance_ratio_ < 0) {
       const int edit_distance = EditDistance();
-      edit_distance_ratio_ =
-          1.0 - static_cast<double>(edit_distance) * 2 /
-                    (first_string_.size() + second_string_.size());
+      edit_distance_ratio_ = std::max(
+          0.0, 1.0 - static_cast<double>(edit_distance) * 2 /
+                         (first_string_.size() + second_string_.size()));
     }
     return edit_distance_ratio_;
   }

chrome/common/string_matching/sequence_matcher_unittest.cc

@@ -127,5 +127,11 @@ TEST_F(SequenceMatcherTest, TestEditDistanceRatio) {
                               base::UTF8ToUTF16("cats white"), true)
                   .Ratio(),
               0.2, 0.01);
+
+  // Totally different
+  EXPECT_NEAR(SequenceMatcher(base::UTF8ToUTF16("dog"),
+                              base::UTF8ToUTF16("elphant"), true)
+                  .Ratio(),
+              0.0, 0.01);
 }
 }  // namespace