[histogram] Make histograms more resistant to overflows.

Instead of calculating deltas from cumulative snapshots, store unlogged
histogram samples in a separate HistogramSample object.
This will reduce overflows from long-running sessions given that data
between UMA pings fits into 32-bit integer.

BUG=719446

Review-Url: https://codereview.chromium.org/2867303004
Cr-Commit-Position: refs/heads/master@{#471359}
7 files changed