[ios] Allow Finch-controlled reduction of persistent session size

This CL allows to change max number of stored and restored navigation
items via Finch within [40, 75] range. This experiment will allow to
understand if reducing stored session size can reduce out of memory
crashes.

Bug: 1115279
Change-Id: I2a4c40f6949631df4eaa503f21b909cd0c3994ac
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2493120Reviewed-by: Justin Cohen <justincohen@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Auto-Submit: Eugene But <eugenebut@chromium.org>
Cr-Commit-Position: refs/heads/master@{#820334}

ios/web/common/features.h

@@ -10,6 +10,10 @@
 namespace web {
 namespace features {
 
+// Reduces the size of the session to persist when enabled. Specific size is
+// obtained from "session-size" Finch parameter.
+extern const base::Feature kReduceSessionSize;
+
 // Used to crash the browser if unexpected URL change is detected.
 // https://crbug.com/841105.
 extern const base::Feature kCrashOnUnexpectedURLChange;

ios/web/common/features.mm

@@ -11,6 +11,9 @@
 namespace web {
 namespace features {
 
+const base::Feature kReduceSessionSize{"ReduceSessionSize",
+                                       base::FEATURE_DISABLED_BY_DEFAULT};
+
 const base::Feature kCrashOnUnexpectedURLChange{
     "CrashOnUnexpectedURLChange", base::FEATURE_ENABLED_BY_DEFAULT};
 

ios/web/navigation/wk_navigation_util.mm

@@ -40,9 +40,22 @@ int GetSafeItemRange(int last_committed_item_index,
                      int item_count,
                      int* offset,
                      int* size) {
-  *size = std::min(kMaxSessionSize, item_count);
-  *offset = std::min(last_committed_item_index - kMaxSessionSize / 2,
-                     item_count - kMaxSessionSize);
+  int max_session_size = kMaxSessionSize;
+  if (base::FeatureList::IsEnabled(features::kReduceSessionSize)) {
+    if (@available(iOS 14.0, *)) {
+      // IOS.MetricKit.ForegroundExitData is supported starting from iOS 14, and
+      // it's the only good metric to track effect of the session size on OOM
+      // crashes.
+      max_session_size = base::GetFieldTrialParamByFeatureAsInt(
+          features::kReduceSessionSize, "session-size", kMaxSessionSize);
+      max_session_size = MIN(max_session_size, kMaxSessionSize);
+      max_session_size = MAX(max_session_size, 40);
+    }
+  }
+
+  *size = std::min(max_session_size, item_count);
+  *offset = std::min(last_committed_item_index - max_session_size / 2,
+                     item_count - max_session_size);
   *offset = std::max(*offset, 0);
   return last_committed_item_index - *offset;
 }