[ntp][modules] Log max ratio of a module visible to the user

The max ratio is logged upon NTP unload and has perdecage (x/10th)
granularity.

The max ratio is logged with the chrome.metricsPrivate API, which this
CL enables on chrome://new-tab-page. Other NTP metrics are logged via
custom renderer -> browser Mojo calls. However, we decided to employ
chrome.privateMetrics in this CL since there is a potential race
condition between page unload and the custom Mojo calls. In the future
we will likely migrate our other metrics to chrome.privateMetrics as
well.

Bug: 1166865
Change-Id: I32f98b0cdb7caaab25194c556bff9911f0b1c116
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2631201
Reviewed-by: Moe Ahmadi <mahmadi@chromium.org>
Reviewed-by: Istiaque Ahmed <lazyboy@chromium.org>
Reviewed-by: Mark Pearson <mpearson@chromium.org>
Auto-Submit: Tibor Goldschwendt <tiborg@chromium.org>
Commit-Queue: Tibor Goldschwendt <tiborg@chromium.org>
Cr-Commit-Position: refs/heads/master@{#847335}