Add support for sparse histograms to WebKitPlatformSupportImpl

webkit/glue/webkitplatformsupport_impl.cc

Index: webkit/glue/webkitplatformsupport_impl.cc
diff --git a/webkit/glue/webkitplatformsupport_impl.cc b/webkit/glue/webkitplatformsupport_impl.cc
index 5c8127960645a165a080c422dc17fdd620fc4cb8..da5fd32427e33e0baf22f42fbd32c39374423f01 100644
--- a/webkit/glue/webkitplatformsupport_impl.cc
+++ b/webkit/glue/webkitplatformsupport_impl.cc
@@ -15,6 +15,7 @@
 #include "base/memory/singleton.h"
 #include "base/message_loop.h"
 #include "base/metrics/histogram.h"
+#include "base/metrics/sparse_histogram.h"
 #include "base/metrics/stats_counters.h"
 #include "base/platform_file.h"
 #include "base/process_util.h"
@@ -443,6 +444,10 @@ void WebKitPlatformSupportImpl::histogramEnumeration(
   counter->Add(sample);
 }
 
+void WebKitPlatformSupportImpl::histogramSparse(const char* name, int sample) {
+  UMA_HISTOGRAM_SPARSE_SLOWLY(name, sample);

[jar (doing other things), 2013/07/19 17:55:28]

nit: This works perfectly... but it scared me, since you can't use the UMA
histogram macros (usually) when the name is variable.  Can you add the comment:

// For sparse histograms, we can use the macro, as it does not incorporate a
static.



FWIW: We should not use a macro definition for this, and the sparse_histogram.h
file should instead define a *function* in this case.  I didn't see the review,
or I would have pointed out that we prefer functions, and only use macros when
we are forced to.  Oh well.

[ajuma, 2013/07/19 18:37:21]

Done.

+}
+
 const unsigned char* WebKitPlatformSupportImpl::getTraceCategoryEnabledFlag(
     const char* category_group) {
   return TRACE_EVENT_API_GET_CATEGORY_GROUP_ENABLED(category_group);