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This organizes the Safe Browsing interstitial histograms in a way that
should scale better as new warnings or buttons are added.

Plus: all of the recording is done from within the command handling
method, instead of in the secondary methods like OnProceed.

BUG=

Review URL: https://codereview.chromium.org/576653002

Cr-Commit-Position: refs/heads/master@{#295294}