Stats traversal algorithm added.

This is part of the work to add a selector argument to getStats().

Changes:
- TakeReferencedStats() added, which traverses the stats graph and takes
  any stats from the report that are directly or indirectly accessible
  from the starting stats objects in the stats graph. The result is
  returned as a stats report.
- GetStatsReferencedIds(), an efficient helper function for getting
  neighbor stats object IDs.
- RTCStatsReport::Take(), removed the stats object with the given ID and
  returns ownership of it (so that it can be added to another report).

TakeReferencedStats() is tested with a bunch of sample stats graphs.

GetStatsReferencedIds() is tested in the rtcstats_integrationttest.cc,
making sure the expected IDs are returned. The expected IDs are the
values of the stats object members with the "Id" or "Ids" suffix.

Design doc:
https://docs.google.com/document/d/18BywbtXgHCjsbR5nWBedpzqDjAfXrFSTJNiADnzoK0w/edit?usp=sharing

Bug: chromium:680172
Change-Id: I5da9da8250da0cb05adb864015901393a4290776
Reviewed-on: https://webrtc-review.googlesource.com/60869
Commit-Queue: Henrik Boström <hbos@webrtc.org>
Reviewed-by: Taylor Brandstetter <deadbeef@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22381}

stats/rtcstatsreport.cc

@@ -84,6 +84,15 @@ const RTCStats* RTCStatsReport::Get(const std::string& id) const {
   return nullptr;
 }
 
+std::unique_ptr<const RTCStats> RTCStatsReport::Take(const std::string& id) {
+  StatsMap::iterator it = stats_.find(id);
+  if (it == stats_.end())
+    return nullptr;
+  std::unique_ptr<const RTCStats> stats = std::move(it->second);
+  stats_.erase(it);
+  return stats;
+}
+
 void RTCStatsReport::TakeMembersFrom(
     rtc::scoped_refptr<RTCStatsReport> victim) {
   for (StatsMap::iterator it = victim->stats_.begin();

stats/rtcstatsreport_unittest.cc

@@ -110,6 +110,19 @@ TEST(RTCStatsReport, StatsOrder) {
   EXPECT_EQ(i, static_cast<int64_t>(7));
 }
 
+TEST(RTCStatsReport, Take) {
+  rtc::scoped_refptr<RTCStatsReport> report = RTCStatsReport::Create(0);
+  report->AddStats(std::unique_ptr<RTCStats>(new RTCTestStats1("A", 1)));
+  report->AddStats(std::unique_ptr<RTCStats>(new RTCTestStats1("B", 2)));
+  EXPECT_TRUE(report->Get("A"));
+  EXPECT_EQ(report->size(), 2u);
+  auto a = report->Take("A");
+  EXPECT_TRUE(a);
+  EXPECT_EQ(report->size(), 1u);
+  EXPECT_FALSE(report->Get("A"));
+  EXPECT_FALSE(report->Take("A"));
+}
+
 TEST(RTCStatsReport, TakeMembersFrom) {
   rtc::scoped_refptr<RTCStatsReport> a = RTCStatsReport::Create(1337);
   EXPECT_EQ(a->timestamp_us(), 1337u);