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Currently the number of pages per bucket is limited to 2^16,
and this has caused a bunch of crash reports at partitionBucketFull().

This CL increase the limit to 2^24 instead of decreasing the limit of
|numSystemPagesPerSlotSpan| to 2^8 (It is statically guaranteed that |numSystemPagesPerSlotSpan|
does not exceed 2^8). As a result, this CL doesn't change sizeof(PartitionBucket).

BUG=87772

Review URL: https://codereview.chromium.org/1622553004

Cr-Commit-Position: refs/heads/master@{#371745}