Fix ScoreTabsWithPairwiseScorer's comparator

This fixes a crash when building with _LIBCPP_DEBUG=0. This function
returned true for nullopt < nullopt when it should return false.

Bug: 923166
Change-Id: I904566ad19ca4ac6e1405db15851ffa79b3a4836
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1894099
Auto-Submit: David Benjamin <davidben@chromium.org>
Commit-Queue: Charles . <charleszhao@chromium.org>
Reviewed-by: Charles . <charleszhao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#711530}

chrome/browser/resource_coordinator/tab_ranker/tab_score_predictor.cc

@@ -234,10 +234,10 @@ std::map<int32_t, float> TabScorePredictor::ScoreTabsWithPairwiseScorer(
             [&tabs](const int32_t id1, const int32_t id2) {
               const auto& tab1 = tabs.at(id1);
               const auto& tab2 = tabs.at(id2);
-              if (!tab1)
-                return true;
               if (!tab2)
                 return false;
+              if (!tab1)
+                return true;
               return tab1->mru_index < tab2->mru_index;
             });