histograms.xml: Added a new cryptohome checksum enum value.
histograms.xml: Added a new cryptohome checksum enum value.

BUG=chromium:411923
TEST=validate_format, pretty_print

Review URL: https://codereview.chromium.org/1576543002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#368474}

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