[Cronet] Make HistogramManager public APIs thread-safe.

CronetEngine.getGlobalMetricsDeltas() exposes the underlying
HistogramManager public APIs on multiple threads, so crashes
can occur if accessed concurrently. The current public APIs
are GetInstance() which simply accesses a thread-safe
LazyInstance, and GetDeltas() which doesn't look thread-safe
so I've added a Lock.

BUG=603028

Review URL: https://codereview.chromium.org/1882563004

Cr-Commit-Position: refs/heads/master@{#387009}

components/cronet/histogram_manager.cc

@@ -53,18 +53,21 @@ void HistogramManager::InconsistencyDetectedInLoggedCount(int amount) {
 }
 
 bool HistogramManager::GetDeltas(std::vector<uint8_t>* data) {
-  // Clear the protobuf between calls.
-  uma_proto_.Clear();
-  // "false" to StatisticsRecorder::begin() indicates to *not* include
-  // histograms held in persistent storage on the assumption that they will be
-  // visible to the recipient through other means.
-  histogram_snapshot_manager_.PrepareDeltas(
-      base::StatisticsRecorder::begin(false), base::StatisticsRecorder::end(),
-      base::Histogram::kNoFlags, base::Histogram::kUmaTargetedHistogramFlag);
-  int32_t data_size = uma_proto_.ByteSize();
-  data->resize(data_size);
-  if (uma_proto_.SerializeToArray(&(*data)[0], data_size))
-    return true;
+  if (get_deltas_lock_.Try()) {
+    base::AutoLock lock(get_deltas_lock_, base::AutoLock::AlreadyAcquired());
+    // Clear the protobuf between calls.
+    uma_proto_.Clear();
+    // "false" to StatisticsRecorder::begin() indicates to *not* include
+    // histograms held in persistent storage on the assumption that they will be
+    // visible to the recipient through other means.
+    histogram_snapshot_manager_.PrepareDeltas(
+        base::StatisticsRecorder::begin(false), base::StatisticsRecorder::end(),
+        base::Histogram::kNoFlags, base::Histogram::kUmaTargetedHistogramFlag);
+    int32_t data_size = uma_proto_.ByteSize();
+    data->resize(data_size);
+    if (uma_proto_.SerializeToArray(&(*data)[0], data_size))
+      return true;
+  }
   data->clear();
   return false;
 }

components/cronet/histogram_manager.h

@@ -15,13 +15,14 @@
 #include "base/memory/scoped_ptr.h"
 #include "base/metrics/histogram_flattener.h"
 #include "base/metrics/histogram_snapshot_manager.h"
+#include "base/synchronization/lock.h"
 #include "components/metrics/proto/chrome_user_metrics_extension.pb.h"
 
 namespace cronet {
 
 // A HistogramManager instance is created by the app. It is the central
 // controller for the acquisition of log data, and recording deltas for
-// transmission to an external server.
+// transmission to an external server. Public APIs are all thread-safe.
 class HistogramManager : public base::HistogramFlattener {
  public:
   HistogramManager();
@@ -52,6 +53,10 @@ class HistogramManager : public base::HistogramFlattener {
   // Stores the protocol buffer representation for this log.
   metrics::ChromeUserMetricsExtension uma_proto_;
 
+  // Should be acquired whenever GetDeltas() is executing to maintain
+  // thread-safety.
+  base::Lock get_deltas_lock_;
+
   DISALLOW_COPY_AND_ASSIGN(HistogramManager);
 };