Add renderer launch count to stability metrics.

This will make it possible to have a more meaningful measure of true extension
renderer stability as currently there is no denominator that can be used for
normalization of extension stability.

Desktop/Android only.

BUG=621120

Review-Url: https://codereview.chromium.org/2130753004
Cr-Commit-Position: refs/heads/master@{#405851}
6 files changed