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The UMA was added to get data on how often we need to serialize ops >
512k and what the size distribution for these ops looks like. The data
from stable channels puts it at ~0.4 ops per million tiles, and even in
those cases ~85% of them fit within 600k and only 0.01% of the cases
exceeded the max limit of 16M. So it looks like this is enough of an
edge case that the current solution for reallocation on a serialization
failure is good enough.

This change removes the UMA since its no longer needed and increments the
default size we begin with to 600k so the common cases require minimum
reallocations.

R=enne@chromium.org, piman@chromium.org

Change-Id: I022ead6728c2b0a04bc898db892d3ccc4ac5d0e9
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1641810
Commit-Queue: Khushal <khushalsagar@chromium.org>
Reviewed-by: Antoine Labour <piman@chromium.org>
Reviewed-by: enne <enne@chromium.org>
Reviewed-by: Steven Holte <holte@chromium.org>
Cr-Commit-Position: refs/heads/master@{#666025}