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Continuation of crrev.com/c/2455947 and crrev.com/c/2463043.

The remaining GetAllocatedSize() was still ambiguous, returning two
different things only one of which could be considered "allocated size".
Furthermore, naming of function params and local variables wasn't always
clear.

This is an attempt to make the terminology consistent:
- requested_size - what the app requested
- extras - PA-internal data surrounding the allocated data, like
  tag/ref-count (for CheckedPtr) or cookies (debug only)
- raw_size - requested_size + extras
- usable_size (>=requested_size) - in case of over-allocation, this is
  what's available for the app (no risk of overriding extras, etc.)
- utilized_slot_size - contiguous region of the slot that accommodates
  all of the above (requested allocation, additional usable space and
  extras); equal to raw_size if it's possible to save it in meta-data
  (for large allocations only), slot_size otherwise

Change-Id: Ia2a771af6d29261b049b0dc98fede6395dc7a35f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2462909
Commit-Queue: Bartek Nowierski <bartekn@chromium.org>
Auto-Submit: Bartek Nowierski <bartekn@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#816469}