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zlib uses malloc() to allocate temporary data. Unfortunately, on Android,
jemalloc (malloc()'s underlying implementation) caches temporary data after
free() in a thread-local cache. As renderers don't allocate from malloc() often
in background threads, this data is not reclaimed. This increases memory usage
by up to 256kiB per background compression thread, which is the cause for a
sizable malloc() memory regression from foreground string compression (see
linked bug).

The deeper reason for the regression is more complex, see details in the linked
bug, and
https://docs.google.com/document/d/1aRIifaHF5l9relq9vHirTOWt2AZoAkDNqHiYZrWGE9A/edit?usp=sharing

It involves some interaction between:
- jemalloc implementation and memory accounting
- Android's configuration of jemalloc
- Chrome's allocation and threading patterns
- zlib's allocation patterns

Nevertheless, the regression is real, and to mitigate it, use PartitionAlloc to
allocate zlib's temporary data.

This is not necessary for decompression, as the allocation patterns are not the
same, and the main thread doesn't have the same issues. As such, only do it for
compression, to avoid a needlessly complex CL.

On PartitionAlloc vs malloc():
The only path using partition alloc in this CL in from the renderer process,
where it is already widely used. The issue is less likely to appear in the
browser process as malloc() is more widely used there, though further
investigation may reveal issues.

Android vs everywhere:
The issue is present at least with jemalloc(), that is Android. However thread
caches and arena-allocators are used on other platforms as well, and as the
allocation is not performance-sensitive, using PartitionAlloc everwhere makes
the code simpler, and protects against potential issues (as we know that this
specific issue does not arise with PartitionAlloc).

Bug: 931553, 924164
Change-Id: I9e94e9cea5d51fac67b04fdb1681427e57bfbe1f
Reviewed-on: https://chromium-review.googlesource.com/c/1472578
Commit-Queue: Benoit L <lizeb@chromium.org>
Reviewed-by: Alexei Svitkine <asvitkine@chromium.org>
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Cr-Commit-Position: refs/heads/master@{#632372}