WebView: Enable the persistent histogram by LocalMemory to be default.

- Add a new parameter default_local_memory in
InstantiatePersistentHistograms()
- Set it true for WebView.
- Plan to merge this patch to M86.

Bug: 1090682
Change-Id: I218c9960c9cb72e184e36f78e97096a8f6691759
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2411706
Commit-Queue: Michael Bai <michaelbai@chromium.org>
Reviewed-by: Brian White <bcwhite@chromium.org>
Cr-Commit-Position: refs/heads/master@{#807583}

android_webview/browser/aw_field_trials.cc

@@ -12,11 +12,10 @@
 
 void AwFieldTrials::SetupFieldTrials() {
   // https://crbug.com/1093420: enable for persistent metrics.
-  if (base::FeatureList::IsEnabled(base::kPersistentHistogramsFeature)) {
   // Persistent histograms must be enabled as soon as possible.
   base::FilePath metrics_dir;
   if (base::PathService::Get(base::DIR_ANDROID_APP_DATA, &metrics_dir)) {
-      InstantiatePersistentHistograms(metrics_dir);
-    }
+    InstantiatePersistentHistograms(metrics_dir,
+                                    /*default_local_memory=*/true);
   }
 }

components/metrics/persistent_histograms.cc

@@ -84,7 +84,8 @@ const char kBrowserMetricsName[] = "BrowserMetrics";
 
 // Check for feature enabling the use of persistent histogram storage and
 // enable the global allocator if so.
-void InstantiatePersistentHistograms(const base::FilePath& metrics_dir) {
+void InstantiatePersistentHistograms(const base::FilePath& metrics_dir,
+                                     bool default_local_memory) {
   // Create a directory for storing completed metrics files. Files in this
   // directory must have embedded system profiles. If the directory can't be
   // created, the file will just be deleted below.
@@ -148,7 +149,7 @@ void InstantiatePersistentHistograms(const base::FilePath& metrics_dir) {
     storage = kLocalMemory;
 
   // Create a global histogram allocator using the desired storage type.
-  if (storage.empty() || storage == kMappedFile) {
+  if (storage == kMappedFile || (storage.empty() && !default_local_memory)) {
     if (!base::PathExists(upload_dir)) {
       // Handle failure to create the directory.
       result = kNoUploadDir;
@@ -177,7 +178,8 @@ void InstantiatePersistentHistograms(const base::FilePath& metrics_dir) {
                            &base::GlobalHistogramAllocator::CreateSpareFile),
                        std::move(spare_file), kAllocSize),
         base::TimeDelta::FromSeconds(kSpareFileCreateDelaySeconds));
-  } else if (storage == kLocalMemory) {
+  } else if (storage == kLocalMemory ||
+             (default_local_memory && storage.empty())) {
     // Use local memory for storage even though it will not persist across
     // an unclean shutdown. This sets the result but the actual creation is
     // done below.

components/metrics/persistent_histograms.h

@@ -15,6 +15,7 @@
 extern const char kBrowserMetricsName[];
 
 // Do all the checking and work necessary to enable persistent histograms.
-void InstantiatePersistentHistograms(const base::FilePath& metrics_dir);
+void InstantiatePersistentHistograms(const base::FilePath& metrics_dir,
+                                     bool default_local_memory = false);
 
 #endif  // COMPONENTS_METRICS_PERSISTENT_HISTOGRAMS_H_