base/allocator: Don't zero already zeroed allocations in PartitionAlloc.

This was already attempted in the past, but had to be reverted due to
failures on macOS (see crbug.com/890752). This is because pages decommit
is not synchronous on macOS, but it is on other platorms.

The revert actually cost more performance than was necessary, as it
affected all platforms, and also newly allocated pages, which are
guaranteed to be zeroed on all operating systems. As a consequence,
allocating a large ArrayBuffer for instance can be pretty slow (and even
crash), since the memory is expliclty zeroed at the allocation site.

This CL adds the simplest fix:
- Newly allocated pages are never needlessly zeroed
- Recommitted ones are only zeroed on macOS

The expected impact is:
- Faster large allocations (and perhaps less crashy, at least on systems
  such as Linux/Android with overcommit), including ArrayBuffers.
- Faster allocations of zeroed memory on !macOS systems.

Bug: 864462, 890752, 998048
Change-Id: I4e3d7d8d63ecea41923db3b9ae8613719df7c712
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2308419
Reviewed-by: Kentaro Hara <haraken@chromium.org>
Commit-Queue: Benoit L <lizeb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#790417}