[blink] Remove no longer used blink::EnumerationHistogram helper.

Usage has been migrated to use the standard //base/metrics helpers.

Bug: 1047547
Change-Id: I8edfefb774a6154e19763833a7885e491d9a3996
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2559557Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Cr-Commit-Position: refs/heads/master@{#830912}

third_party/blink/renderer/platform/instrumentation/histogram.cc

@@ -41,16 +41,6 @@ void CustomCountHistogram::CountMilliseconds(base::TimeDelta delta) {
       delta.InMilliseconds()));
 }
 
-EnumerationHistogram::EnumerationHistogram(
-    const char* name,
-    base::HistogramBase::Sample boundary_value)
-    : CustomCountHistogram(base::LinearHistogram::FactoryGet(
-          name,
-          1,
-          boundary_value,
-          boundary_value + 1,
-          base::HistogramBase::kUmaTargetedHistogramFlag)) {}
-
 LinearHistogram::LinearHistogram(const char* name,
                                  base::HistogramBase::Sample min,
                                  base::HistogramBase::Sample max,

third_party/blink/renderer/platform/instrumentation/histogram.h

@@ -39,13 +39,6 @@ class PLATFORM_EXPORT CustomCountHistogram {
   base::HistogramBase* histogram_;
 };
 
-class PLATFORM_EXPORT EnumerationHistogram : public CustomCountHistogram {
- public:
-  // |boundaryValue| must be strictly greater than samples passed to |count|.
-  EnumerationHistogram(const char* name,
-                       base::HistogramBase::Sample boundary_value);
-};
-
 class PLATFORM_EXPORT LinearHistogram : public CustomCountHistogram {
  public:
   explicit LinearHistogram(const char* name,