Remove prerender metrics observer

This is not being used in actual production. This CL
- Removes the observer
- Marks the corresponding histograms as obsolete.
None of these are being recorded as per
 http://shortn/_87vkPT3VpS
- Removes some of the browsertests

Change-Id: I4c79538854076dfa2b990578ce1a3898a87365d0
Bug: 778222
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1496657
Reviewed-by: Egor Pasko <pasko@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Ryan Sturm <ryansturm@chromium.org>
Commit-Queue: Tarun Bansal <tbansal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#637848}

chrome/browser/BUILD.gn

@@ -1009,8 +1009,6 @@ jumbo_split_static_library("browser") {
     "page_load_metrics/observers/omnibox_suggestion_used_page_load_metrics_observer.h",
     "page_load_metrics/observers/page_capping_page_load_metrics_observer.cc",
     "page_load_metrics/observers/page_capping_page_load_metrics_observer.h",
-    "page_load_metrics/observers/prerender_page_load_metrics_observer.cc",
-    "page_load_metrics/observers/prerender_page_load_metrics_observer.h",
     "page_load_metrics/observers/previews_lite_page_redirect_metrics_observer.cc",
     "page_load_metrics/observers/previews_lite_page_redirect_metrics_observer.h",
     "page_load_metrics/observers/previews_page_load_metrics_observer.cc",

chrome/browser/page_load_metrics/page_load_metrics_initialize.cc

@@ -33,7 +33,6 @@
 #include "chrome/browser/page_load_metrics/observers/offline_page_previews_page_load_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/omnibox_suggestion_used_page_load_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/page_capping_page_load_metrics_observer.h"
-#include "chrome/browser/page_load_metrics/observers/prerender_page_load_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/previews_lite_page_redirect_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/previews_page_load_metrics_observer.h"
 #include "chrome/browser/page_load_metrics/observers/previews_ukm_observer.h"
@@ -168,12 +167,6 @@ void PageLoadMetricsEmbedder::RegisterObservers(
         std::make_unique<LocalNetworkRequestsPageLoadMetricsObserver>());
     tracker->AddObserver(
         std::make_unique<StaleWhileRevalidatePageLoadMetricsObserver>());
-  } else {
-    std::unique_ptr<page_load_metrics::PageLoadMetricsObserver>
-        prerender_observer =
-            PrerenderPageLoadMetricsObserver::CreateIfNeeded(web_contents_);
-    if (prerender_observer)
-      tracker->AddObserver(std::move(prerender_observer));
   }
   tracker->AddObserver(
       std::make_unique<OmniboxSuggestionUsedMetricsObserver>(IsPrerendering()));