Add a description of Sync.LocalDataFailedToLoad histogram.
Add a description of Sync.LocalDataFailedToLoad histogram.

We need it to analyse the spikes of LoginDatabase failures recently.

BUG=419070

Review URL: https://codereview.chromium.org/1234873007

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#339397}

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