Prerender: Remove PerceivedPLT histograms
Reasons:
* We stopped looking at this class of metrics
* It saves 5320 bytes of the code size on Android (see below)
* Saves multi-GiB of data on UMA servers
Removing PerceivedPLT also made a bit of state unused, hence it was removed:
* PrerenderHistograms objects are now almost stateless
* PrerenderTabHelper now only manages origin_, url_ and swap_ticks_
To confirm the binary size change:
1. build chrome_apk with is_official_build = true
2. shell> find out/ReleaseOfficial/gen -name libchrome.so -exec size {} \;
BUG=698733,698956,603203
Review-Url: https://codereview.chromium.org/2738783002
Cr-Commit-Position: refs/heads/master@{#461105}
8 files changed
