Add UMA to track app install banner outcomes.
It is difficult to get a picture for what is happening when banners are
shown, as three separate metrics track a number of fine-grained
outcomes.
This CL adds a new UMA solely tracking the user response to a banner,
that is, once a banner is shown, whether users accept, dismiss, or
ignore it. Native and web apps are tracked by separate metrics.
BUG=516956
Review URL: https://codereview.chromium.org/
1269303002
Cr-Commit-Position: refs/heads/master@{#342270}