feat: add memory to app.getAppMetrics()

[miniak]

Description of Change

Bring back memory info per process returned by app.getAppMetrics(). Follow-up to #18718.
It does not depend on Chromium internals, it’s calling the OS API directly.

Checklist

• PR description included and stakeholders cc'd
• npm test passes
• tests are changed or added
• relevant documentation is changed or added
• PR title follows semantic commit guidelines
• PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: Added memory to app.getAppMetrics().

[MarshallOfSound]

Just gonna have to ask the hard question here, what dependencies does this API have on Chromium internals / content APIs. We've been hurt twice now by memory APIs being removed internally in Chromium, if this one could potentially have the same issue we should seriously consider not adding it.

[miniak]

Just gonna have to ask the hard question here, what dependencies does this API have on Chromium internals / content APIs. We've been hurt twice now by memory APIs being removed internally in Chromium, if this one could potentially have the same issue we should seriously consider not adding it.

@MarshallOfSound Added more details to the description. We've been affected by the removal of memory info due to the Chromium changes as our telemetry depends on it heavily.

[zcbenz]

Compilation on Windows fails:

../../electron/atom/browser/api/process_metric.cc(73,34): error: out-of-line definition of 'K32GetProcessMemoryInfo' does not match any declaration in 'atom::ProcessMetric'; did you mean 'GetProcessMemoryInfo'?
17375ProcessMemoryInfo ProcessMetric::GetProcessMemoryInfo() const {
17376                                 ^~~~~~~~~~~~~~~~~~~~
17377                                 GetProcessMemoryInfo
17378

[ckerr]

It would be preferable to support this on Linux, too.

[miniak]

@ckerr can you please check the Linux version?

[miniak]

@zcbenz can you please check again?

[ckerr]

The scaffolding, tests, doc changes, and Linux code all LGTM. Tested on Ubuntu 17.04 👍

The Mac and Windows changes look sane too but I haven't tested them personally. If someone else wants to also give this a pass, that would be great.

[codebytere]

(approval given that all of the issues i would have raised have all been addressed)

[release-clerk]

Release Notes Persisted

Added memory to app.getAppMetrics().