[bfcache] Add a feature to disable HTTP cache for history navigations

This will allow us to measure the performance difference and see
the difference in the user engagement metrics and extrapolate to
estimate the usefulness of bfcache.

One-pager: https://docs.google.com/document/d/10Ff_5oq7k-oYuXTTKdN0v_JKG3bU1GPgAWWF7i4nMvs/edit#

R=creis@chromium.org
BUG=954271

Change-Id: I54dfe4634739a6c1eb3d33eee5025ecfe8622945
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1572343
Commit-Queue: Alexander Timin <altimin@chromium.org>
Reviewed-by: Charlie Reis <creis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#652374}

content/browser/frame_host/navigation_request.cc

@@ -10,8 +10,10 @@
 #include "base/bind_helpers.h"
 #include "base/debug/alias.h"
 #include "base/debug/dump_without_crashing.h"
+#include "base/feature_list.h"
 #include "base/metrics/field_trial_params.h"
 #include "base/optional.h"
+#include "base/rand_util.h"
 #include "base/strings/string_util.h"
 #include "base/strings/stringprintf.h"
 #include "base/task/post_task.h"
@@ -90,6 +92,15 @@ namespace content {
 
 namespace {
 
+// crbug.com/954271: This feature is a part of an ablation study which makes
+// history navigations slower.
+// TODO(altimin): Clean this up after the study finishes.
+constexpr base::Feature kHistoryNavigationDoNotUseCacheAblationStudy{
+    "HistoryNavigationDoNotUseCacheAblationStudy",
+    base::FEATURE_DISABLED_BY_DEFAULT};
+constexpr base::FeatureParam<double> kDoNotUseCacheProbability{
+    &kHistoryNavigationDoNotUseCacheAblationStudy, "probability", 0.0};
+
 // Returns the net load flags to use based on the navigation type.
 // TODO(clamy): Remove the blink code that sets the caching flags when
 // PlzNavigate launches.
@@ -122,6 +133,10 @@ void UpdateLoadFlagsWithCacheFlags(
       if (is_post) {
         *load_flags |=
             net::LOAD_ONLY_FROM_CACHE | net::LOAD_SKIP_CACHE_VALIDATION;
+      } else if (base::FeatureList::IsEnabled(
+                     kHistoryNavigationDoNotUseCacheAblationStudy) &&
+                 base::RandDouble() < kDoNotUseCacheProbability.Get()) {
+        *load_flags |= net::LOAD_BYPASS_CACHE;
       } else {
         *load_flags |= net::LOAD_SKIP_CACHE_VALIDATION;
       }