We can get total and free memory for the entire system using base::SysInfo.

metrics_memory_details.cc contains a way to get the private memory used by each Chrome process.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/1cfe9539a19cb6cdea82c91f78f9341418fa2d13

commit 1cfe9539a19cb6cdea82c91f78f9341418fa2d13
Author: mwlodar <mwlodar@google.com>
Date: Tue Oct 04 22:19:53 2016

Additional fields added to Memory Leak Detector proto

Memory Leak Detector proto gets enriched with a message
for collecting memory usage information from OS
and fields describing the last increasing allocation trend for
the suspected size and call stack.

BUG= chromium:650347 

Review-Url: https://codereview.chromium.org/2391893002
Cr-Commit-Position: refs/heads/master@{#422963}

[modify] https://crrev.com/1cfe9539a19cb6cdea82c91f78f9341418fa2d13/components/metrics/proto/memory_leak_report.proto

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ee4005181ef90e80428b3569fd5a364f50f334e6

commit ee4005181ef90e80428b3569fd5a364f50f334e6
Author: mwlodar <mwlodar@google.com>
Date: Fri Oct 14 00:11:52 2016

Leak reports collect information about the memory usage

LeakDetectorController calls the MetricsMemoryDetails
to collect information about the memory usage and writes
it in the received leak reports.

BUG= 650347 
TEST=1. Modify kDefaultProbability and kDefaultNumProcessesEnabled in leak_detector_controller.cc to always launch the detector.
2. Add a dummy report in leak_detector.cc before detector->NotifyObservers()
3. Add debug info with LOG(ERROR) in leak_detector_controller.cc:OnMemoryDetailCollectionDone()
4. Launch Chromium with flag enable-features=RuntimeMemoryLeakDetector

Review-Url: https://codereview.chromium.org/2396743002
Cr-Commit-Position: refs/heads/master@{#425212}

[modify] https://crrev.com/ee4005181ef90e80428b3569fd5a364f50f334e6/chrome/browser/metrics/leak_detector/leak_detector_controller.cc
[modify] https://crrev.com/ee4005181ef90e80428b3569fd5a364f50f334e6/chrome/browser/metrics/leak_detector/leak_detector_controller.h
[modify] https://crrev.com/ee4005181ef90e80428b3569fd5a364f50f334e6/chrome/browser/metrics/leak_detector/leak_detector_controller_unittest.cc
[modify] https://crrev.com/ee4005181ef90e80428b3569fd5a364f50f334e6/chrome/browser/metrics/metrics_memory_details.cc
[modify] https://crrev.com/ee4005181ef90e80428b3569fd5a364f50f334e6/chrome/browser/metrics/metrics_memory_details.h