Support saving browser metrics to disk and reading them during next run.

base/metrics/persistent_histogram_allocator.cc

Index: base/metrics/persistent_histogram_allocator.cc
diff --git a/base/metrics/persistent_histogram_allocator.cc b/base/metrics/persistent_histogram_allocator.cc
index 62e8cd6d68baa4ca567e97f3c095ce093969eb1c..87c3b192cb3b89ad54c2fd03262e7bcc0b1ac572 100644
--- a/base/metrics/persistent_histogram_allocator.cc
+++ b/base/metrics/persistent_histogram_allocator.cc
@@ -6,6 +6,7 @@
 
 #include <memory>
 
+#include "base/files/important_file_writer.h"
 #include "base/lazy_instance.h"
 #include "base/logging.h"
 #include "base/memory/ptr_util.h"
@@ -704,6 +705,32 @@ GlobalHistogramAllocator::ReleaseForTesting() {
   return WrapUnique(histogram_allocator);
 };
 
+void GlobalHistogramAllocator::SetPersistentLocation(const FilePath& location) {
+  persistent_location_ = location;
+}
+
+bool GlobalHistogramAllocator::WriteToPersistentLocation() {
+#if defined(OS_NACL)
+  // NACL doesn't support file operations, including ImportantFileWriter.
+  NOTREACHED();
+  return false;
+#else
+  // Stop if no destination is set, perhaps because it has not been enabled.
+  if (persistent_location_.empty())

[Ilya Sherman, 2016/04/20 17:10:33]

I'd still prefer the location to be guaranteed to be set, and to track
enabled/disabled state explicitly.

(I don't actually see a way for the location to be unset, except in case of an
error.  If this is really intended to be error-handling code, then please update
the comment.  And, we should think carefully about what might cause that error
to come up, and whether we're willing to live with simply dropping metrics in
that case.  It certainly seems oddly asymmetric to log an error below, but not
to log one here, if this is error-handling code.)

[bcwhite, 2016/04/22 15:18:04]

Enable/Disable: Done.
It can't be unset (at least by existing code) but there may be paths where it
never gets set at all.  I don't think a log message is necessary but I don't
mind adding one.

+    return false;
+
+  StringPiece contents(static_cast<const char*>(data()), used());
+  if (!ImportantFileWriter::WriteFileAtomically(persistent_location_,
+                                                contents)) {
+    LOG(ERROR) << "Could not write persistent histograms to file: "
+               << persistent_location_.value();
+    return false;
+  }
+
+  return true;
+#endif
+}
+
 GlobalHistogramAllocator::GlobalHistogramAllocator(
     std::unique_ptr<PersistentMemoryAllocator> memory)
     : PersistentHistogramAllocator(std::move(memory)),