Remove legacy blink::BooleanHistogram helper.

All uses have been migrated to use the //base helpers directly instead.

Bug: 1047547
Change-Id: I3a293faba3d894dcf8af420e58159116bb518f74
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2500001
Commit-Queue: Daniel Cheng <dcheng@chromium.org>
Commit-Queue: Jeremy Roman <jbroman@chromium.org>
Auto-Submit: Daniel Cheng <dcheng@chromium.org>
Reviewed-by: Jeremy Roman <jbroman@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820974}

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

@@ -41,11 +41,6 @@ void CustomCountHistogram::CountMilliseconds(base::TimeDelta delta) {
       delta.InMilliseconds()));
 }
 
-BooleanHistogram::BooleanHistogram(const char* name)
-    : CustomCountHistogram(base::BooleanHistogram::FactoryGet(
-          name,
-          base::HistogramBase::kUmaTargetedHistogramFlag)) {}
-
 EnumerationHistogram::EnumerationHistogram(
     const char* name,
     base::HistogramBase::Sample boundary_value)

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

@@ -39,11 +39,6 @@ class PLATFORM_EXPORT CustomCountHistogram {
   base::HistogramBase* histogram_;
 };
 
-class PLATFORM_EXPORT BooleanHistogram : public CustomCountHistogram {
- public:
-  BooleanHistogram(const char* name);
-};
-
 class PLATFORM_EXPORT EnumerationHistogram : public CustomCountHistogram {
  public:
   // |boundaryValue| must be strictly greater than samples passed to |count|.