Change how the quota system computes the poolsize for temporary storage.

Adds a QuotaSettings datastructure and changes how the total poolsize
for temporary storage is determined. This CL adds a public content API
that the embedder must use to inform the quota system of the desired

The old computation: [available_space + current_temp_usage] / 3
The new computation: [total_volume_size - os_overhead] / 3

The new computation is simplified to be a function of the size of the
volume, and no longer takes as input how much chrome is currently using.
This also reduces the time take to initialize the quota system.

To defend against filling the drive, as available space gets "too low",
the calculated per-host quota is clamped to the hosts current usage. This
prevents the writing of new data because quota clients query
GetUsageAndQuota() prior to performing a write. Also the eviction logic
deletes existing data to keep |must_remain_available| disk space free.


Cr-Commit-Position: refs/heads/master@{#438741}
96 files changed