Replace getProcessMemoryInfo API in chromium 67

[alexeykuzmin]

ProcessMetrics::GetMemoryBytes won't be available after an upgrade to Chromium 67+, see #13383.
But those metrics are quite important, so we should try to find a way to keep them.

/cc @miniak, @juturu, @nitsakh

[nitsakh]

Based on the chromium task (https://bugs.chromium.org/p/chromium/issues/detail?id=819289) and related notes, it looks like the details in the electron getProcessMemoryInfo API response provide results are platform specific and mean different on different platforms. These no longer exist in chromium 67. Chromium has moved to using the memory_instrumentation apis, and that's the recommended cross platform way.
There are ways to get platform specific details (process_metric.h), but if just the memory footprint is needed we can use the private_memory_footprint from the instrumentation API.
(Also updating the issue title to reflect the getProcessMemoryInfo API deprecation)

[nitsakh]

/cc @electron/reviewers Request your suggestions for the above PR and the API changes.

[bpasero]

@alexeykuzmin @nitsakh is this still an issue given #14847 landed? I cannot find it in Electron 4.0.x though, so not sure if its 5.0.x only?

[codebytere]

This is completed and released in 5-0-x.