Add in-memory caching of network quality estimates across network changes.

Add in-memory caching of network quality estimates across network changes.

This is the first step towards preserving network quality
estimates across browser session restarts.

User may switch between different networks within
the same browser session (e.g., from Wi-Fi to cellular
and back to Wi-Fi). This change caches the estimates
when the device switches networks. If the user
uses the same network again, the cached estimates
would be used to quickly arrive at quality
estimates.

BUG=509712

Review URL: https://codereview.chromium.org/1144163008

Cr-Commit-Position: refs/heads/master@{#338923}