[Cronet] Make HistogramManager public APIs thread-safe.

components/cronet/histogram_manager.cc

Index: components/cronet/histogram_manager.cc
diff --git a/components/cronet/histogram_manager.cc b/components/cronet/histogram_manager.cc
index 4ec3adffb30b5cf39c9b07f8d0e8aeace32d04ef..3ab21cf45466468d063e5c335c8bb5c453984c99 100644
--- a/components/cronet/histogram_manager.cc
+++ b/components/cronet/histogram_manager.cc
@@ -53,6 +53,7 @@ void HistogramManager::InconsistencyDetectedInLoggedCount(int amount) {
 }
 
 bool HistogramManager::GetDeltas(std::vector<uint8_t>* data) {
+  base::AutoLock lock(lock_);

[bcwhite, 2016/04/13 14:12:52]

It doesn't make sense to make multiple, parallel calls to PrepareDeltas while
passing the begin/end iterators from the StatisticsRecorder.  The second thread
through the method will find an empty (or very small) delta.

I suggest explicit Try/Release of the lock and return immediately if the Try
fails.  And rename the lock to indicate that it applies only to the GetDeltas
method.

[pauljensen, 2016/04/13 14:29:45]

It may not make sense to do this, but we shouldn't crash if Cronet users use
this API in a supported way.
We document this.  See the documentation:
 
https://code.google.com/p/chromium/codesearch#chromium/src/components/cronet/...
Again, it may not make sense, but we shouldn't crash if Cronet users use this
API in a supported way.
Good idea, done.
I'd rather we keep it with a broader usage connotation so that future changes
can use it...rather than fall into the prior inadvertently thread-unsafe
mechanisms.

[bcwhite, 2016/04/13 14:42:18]

If this method were to be called while some "future" method owned the lock, the
Try() would fail (and return) and filling of the protobuf might never happen.

   // Clear the protobuf between calls.
   uma_proto_.Clear();
   // "false" to StatisticsRecorder::begin() indicates to *not* include