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On macOS, measurements for private memory footprint overcount by
faulted pages in anonymous shared memory. To discount for this, this CL touches
all the resident pages in anonymous shared memory, thus making them
faulted as well. This relies on two assumptions:

1) Consumers use shared memory from front to back. Thus, if there are
(N) resident pages, those pages represent the first N * PAGE_SIZE bytes in
the shared memory region.

2) The faulting logic is run shortly before the logic that calculates
phys_footprint, thus ensuring that the discrepancy between faulted and
resident pages is minimal.

The performance penalty is expected to be small.

* Most of the time, we expect the pages to already be resident and faulted,
thus incurring a cache penalty read hit [since we read from each resident
page].

* Rarely, we expect the pages to be resident but not faulted, resulting in
soft faults + cache penalty.

* If assumption (1) is invalid, this will potentially fault some
previously non-resident pages, thus increasing memory usage, without fixing
the accounting.

Bug: 812346
Change-Id: I04e91bc09bf6bdf2f9179dd8d678c92425e98fed
Reviewed-on: https://chromium-review.googlesource.com/973883Reviewed-by: Primiano Tucci <primiano@chromium.org>
Reviewed-by: Albert J. Wong <ajwong@chromium.org>
Commit-Queue: Erik Chen <erikchen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#545086}