Remove AbortsPageLoadMetricsObserver

This was an experimental set of histograms which never gained wide
adoption. Expire the only one which is not already expired.

Change-Id: If02d7a9357d96d2f323738750f41a02879f6baa3
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/3785498
Reviewed-by: Ian Clelland <iclelland@chromium.org>
Commit-Queue: Annie Sullivan <sullivan@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1028347}

chrome/browser/BUILD.gn

@@ -971,8 +971,6 @@ static_library("browser") {
     "page_info/about_this_site_tab_helper.h",
     "page_info/chrome_about_this_site_service_client.cc",
     "page_info/chrome_about_this_site_service_client.h",
-    "page_load_metrics/observers/aborts_page_load_metrics_observer.cc",
-    "page_load_metrics/observers/aborts_page_load_metrics_observer.h",
     "page_load_metrics/observers/core/amp_page_load_metrics_observer.cc",
     "page_load_metrics/observers/core/amp_page_load_metrics_observer.h",
     "page_load_metrics/observers/core/ukm_page_load_metrics_observer.cc",

chrome/browser/page_load_metrics/observers/README.md

@@ -331,8 +331,6 @@ Aborts can occur before or after commit. Observers can track aborts that happen
 
 To determine whether an abort occurred, observers should check whether the abort_type field of the PageLoadExtraInfo structure has a value other than ABORT_NONE. The time until the page load was aborted is available in the time_to_abort field of the PageLoadExtraInfo structure.
 
-[`AbortsPageLoadMetricsObserver`](aborts_page_load_metrics_observer.cc) is an observer that tracks page load abort metrics.
-
 ### Aggregating statistics across multiple page loads
 
 Some observer implementers may wish to track statistics aggregated across multiple page loads. Since a `PageLoadMetricsObserver`’s lifetime is bound to a single page load, a separate object with a longer lifetime must be used to track these statistics.