[Cleanup] Rename UMA_HISTOGRAM_SPARSE_SLOWLY to base::UmaHistogramSparse().
[Cleanup] Rename UMA_HISTOGRAM_SPARSE_SLOWLY to base::UmaHistogramSparse().

R=f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org

Bug: chromium:773850
Change-Id: Idef7b4c821dcac03e095d1400534ddf503a22423
Reviewed-on: https://chromium-review.googlesource.com/828530
Reviewed-by: Mark Mentovai <f1b5a91d4d6ad523f2610114591c007e75d15084@chromium.org>

chromium/crashpad,chromium/crashpad,chromium/crashpad