Extend lifetime of PluginVm setup and launch histograms

In the wider team sync it has been discussed that ideally
we get these metrics for one more year, so this CL extends
their lifetime until end of 2020.

Test: compile
Bug: 1016434
Change-Id: If8babc01ef23778fd69bb2c813b801187906b932
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1878408
Reviewed-by: Alex Ilin <alexilin@chromium.org>
Reviewed-by: Olya Kalitova <okalitova@chromium.org>
Commit-Queue: Alex Oldemeier <aoldemeier@chromium.org>
Cr-Commit-Position: refs/heads/master@{#709026}