content: Add histogram for discardable memory allocation size.

content/child/child_discardable_shared_memory_manager.cc

Index: content/child/child_discardable_shared_memory_manager.cc
diff --git a/content/child/child_discardable_shared_memory_manager.cc b/content/child/child_discardable_shared_memory_manager.cc
index c6990e4a8fcb3cfb3646725cea9222325f51e0a1..2fd3d0ad9d1e36fb2437bc5af577d89f502fc2fb 100644
--- a/content/child/child_discardable_shared_memory_manager.cc
+++ b/content/child/child_discardable_shared_memory_manager.cc
@@ -6,6 +6,7 @@
 
 #include "base/debug/crash_logging.h"
 #include "base/memory/discardable_shared_memory.h"
+#include "base/metrics/histogram.h"
 #include "base/process/process_metrics.h"
 #include "base/strings/string_number_conversions.h"
 #include "base/trace_event/trace_event.h"
@@ -66,6 +67,12 @@ ChildDiscardableSharedMemoryManager::AllocateLockedDiscardableMemory(
 
   DCHECK_NE(size, 0u);
 
+  UMA_HISTOGRAM_CUSTOM_COUNTS("Memory.DiscardableAllocationSize",
+                              size / 1024,  // In KB
+                              0,

[Alexei Svitkine (slow), 2015/03/06 15:49:08]

There is already a default overflow bucket between 0 and the min value you
specify here. So this should be 1.

[reveman, 2015/03/07 23:05:45]

Done.

+                              4 * 1024 * 1024,  // 4 GB
+                              256);

[Alexei Svitkine (slow), 2015/03/06 15:49:08]

Why do you need 256 buckets? This is much more than other histograms and I'm
skeptical you actual need this many, but happy to discuss the details.

[reveman, 2015/03/07 23:05:45]

Reduced it to 50 which is the same as UMA_HISTOGRAM_MEMORY_KB. I'm not sure
that's enough but we can start with that. The majority of samples will be in the
1-1000KB range with some samples anywhere from 1MB to multiple GBs. I'm
interested in both the distribution of samples in the 0-1MB range as well as the
more rare large allocations. Maybe I can use a logarithmic scale..

[reveman, 2015/03/09 00:31:21]

Just realized that the bucket layout is already exponential by default so 50
buckets as in the latest patch should be more than enough. Sorry for the noise..

+
   // Round up to multiple of page size.
   size_t pages = (size + base::GetPageSize() - 1) / base::GetPageSize();