Fix results.html for showing results of flaky tests run multiple iterations

run_web_tests.py --iterations=100 [--exit-after-n-failures=1] is very
useful for running a slightly flaky test [until it fails]. Previously
results.html failed to display actual and expected results for a test
with multiple results like "PASS PASS PASS PASS PASS FAIL".

Now when collecting artifacts of a test, if the normal loop fails to
find any artifact, we'll use the last found artifacts.

Change-Id: I6de2be6eafcd0b0295b21df5b8d69866f55b9c0e
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2259514Reviewed-by: Aleks Totic <atotic@chromium.org>
Commit-Queue: Xianzhu Wang <wangxianzhu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#781400}

third_party/blink/web_tests/fast/harness/results.html

@@ -1126,6 +1126,7 @@ const GUI = {
     for(let [iteration, result] of test.actual.split(' ').entries()){
       if(test.artifacts != null){
         let perIterationArtifact = Object();
+        let lastArtifact = Object();
         for(let [name, paths] of Object.entries(test.artifacts)){
           for(let path of paths){
             const is_retry_artifact_path = (path.startsWith('layout-test-results/retry_') ||
@@ -1136,9 +1137,15 @@ const GUI = {
                 perIterationArtifact[name] = [];
               }
               perIterationArtifact[name].push(path);
+            } else {
+              lastArtifact[name] = path;
             }
           }
         }
+        for (let [name, artifact] of Object.entries(lastArtifact)) {
+          if (!perIterationArtifact.hasOwnProperty(name))
+              perIterationArtifact[name] = [artifact];
+        }
         actual_results.push(GUI.translateArtifactsIntoResult(
             perIterationArtifact, result, test));
       }else{