PMA files are for data collection-
CrashpadMetrics-active.pma should not be removed manually.
This thread provides most of the answers, e.g.
They are created if one activates "Automatically send
usage statistics and crash reports to Google" in Settings.
The "live" (i.e. BrowserMetrics-identifier.pma) file has the data
shown in chrome://histograms.
There is also a link to the source where the metrics are generated in the following functions:
Note the stony silence on the questions regarding the existence of a PMA file viewer.
Think the tmp files are removed when Chrome is shutdown down gracefully, but if it aborts, they are ignored in future runs. According to WinApp2, they can be removed manually.