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Reintroduces the PeakPageMemory (Peak Virtual Memory) and
PeakWorkingSetSize (Virtual Memory High Water Mark) metrics to Linux
builds of Zucchini. Linux is used for benchmarking of ELF and DEX
variants of Zucchini as well as general intergration and regression
testing so it is nice to have these metrics remain in the program.

The method used is based on code in base/process/internal_linux.[h|cc]
however, it avoids the overhead of having to create key value pairs
for all of the metrics so is slightly faster. Moreover, the contents
of internal_linux are intended to be private to base/process so in
order to get these metrics it is necessary to replicate part of the
code.

These were originally removed in:
https://chromium-review.googlesource.com/c/chromium/src/+/951923.

Also fixes some weirdness with "// defined(...)" comments.

Change-Id: Id69f16fa912117f3d3756b425e584c48b9f0fff2
Reviewed-on: https://chromium-review.googlesource.com/963604
Commit-Queue: Calder Kitagawa <ckitagawa@google.com>
Reviewed-by: Samuel Huang <huangs@chromium.org>
Cr-Commit-Position: refs/heads/master@{#543483}