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This CL fixes compilation errors caused when passing const-refs or
non-const-refs as value arguments for the UMA macros.


Before this CL, an attempt to pass a const-ref as a value resulted
in errors like this:

base/metrics/histogram_macros_internal.h:30:5: error: static_assert failed due to requirement 'sizeof(const TestEnum &) == 0' "enumerator must define kMaxValue enumerat
or to use this macro!"
    static_assert(
    ^
base/metrics/histogram_macros_unittest.cc:63:3: note: in instantiation of member function 'base::internal::EnumSizeTraits<const TestEnum &, void>::Count' requested here
  UMA_HISTOGRAM_ENUMERATION("Test.ScopedEnumeration3", value_ref);
  ^
base/metrics/histogram_macros.h:84:7: note: expanded from macro 'UMA_HISTOGRAM_ENUMERATION'
      INTERNAL_UMA_HISTOGRAM_ENUMERATION_DEDUCE_BOUNDARY)               \
      ^
In file included from ../../base/metrics/histogram_macros_unittest.cc:5:
In file included from ../../base/metrics/histogram_macros.h:10:
base/metrics/histogram_macros_internal.h:33:12: error: reference to type 'const TestEnum' requires an initializer
    return Enum();


Bug: none
Change-Id: I875ff783bda12cc30079bcf6188ff843b9b55a0c
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1729593Reviewed-by: Jesse Doherty <jwd@chromium.org>
Commit-Queue: Maksim Ivanov <emaxx@chromium.org>
Cr-Commit-Position: refs/heads/master@{#682898}