Treat PPAPI disconnect as a crash for plugin metrics.

This normalizes the behavior the user sees, which is a
IDS_PLUGIN_CRASHED_PROMPT infobar, with the metrics reported by
PluginMetricsProvider - which was only recording a crash for when
the process terminated with TERMINATION_STATUS_PROCESS_CRASHED or
TERMINATION_STATUS_ABNORMAL_TERMINATION.

Each time the infobar pops, there should now be a plugin crash
recorded in the SystemProfile.

BUG=509139
TEST=manual, see bug

Review URL: https://codereview.chromium.org/1259493003

Cr-Commit-Position: refs/heads/master@{#341566}
3 files changed