Store test placeholders in a set rather than a vector.

Previously, test placeholders were extracted from test_locations_, a std::map,
which implicitly deduplicated the keys. After creating test_placeholders_, I
stored them in a vector. Unfortunately, this causes duplicate entries to occur
when iterations > 1. Storing them in a set returns the previous behavior.

Bug: 934996
Change-Id: I81825b1f95064ac59594b3531cfd8409c631e8f8
Reviewed-on: https://chromium-review.googlesource.com/c/1484074Reviewed-by: Lei Zhang <thestig@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#634868}

base/test/launcher/test_results_tracker.cc

@@ -234,7 +234,7 @@ void TestResultsTracker::AddTestLocation(const std::string& test_name,
 }
 
 void TestResultsTracker::AddTestPlaceholder(const std::string& test_name) {
-  test_placeholders_.push_back(test_name);
+  test_placeholders_.insert(test_name);
 }
 
 void TestResultsTracker::AddTestResult(const TestResult& result) {

base/test/launcher/test_results_tracker.h

@@ -141,7 +141,7 @@ class TestResultsTracker {
   std::map<std::string, CodeLocation> test_locations_;
 
   // Name of tests that will run and produce results.
-  std::vector<std::string> test_placeholders_;
+  std::set<std::string> test_placeholders_;
 
   // Set of all disabled tests in the current executable.
   std::set<std::string> disabled_tests_;