[ios] Add more metrics for reading and write session plist times.

Record the time taken to read a stored session window, or write one to a file.

Bug: 1007337, 1010609
Change-Id: I8f98348c51b8b407b60d9514008431c2fa564d87
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1860303Reviewed-by: Mark Pearson <mpearson@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Commit-Queue: Justin Cohen <justincohen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#706774}

ios/chrome/browser/sessions/session_service_ios.mm

@@ -13,11 +13,13 @@
 #include "base/logging.h"
 #import "base/mac/foundation_util.h"
 #include "base/memory/ref_counted.h"
+#include "base/metrics/histogram_functions.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/sequenced_task_runner.h"
 #include "base/strings/sys_string_conversions.h"
 #include "base/task/post_task.h"
 #include "base/threading/scoped_blocking_call.h"
+#include "base/time/time.h"
 #import "ios/chrome/browser/sessions/session_ios.h"
 #import "ios/chrome/browser/sessions/session_window_ios.h"
 #import "ios/web/public/session/crw_navigation_item_storage.h"
@@ -130,7 +132,11 @@ NSString* const kRootObjectKey = @"root";  // Key for the root object.
 
 - (SessionIOS*)loadSessionFromDirectory:(NSString*)directory {
   NSString* sessionPath = [[self class] sessionPathForDirectory:directory];
-  return [self loadSessionFromPath:sessionPath];
+  base::TimeTicks start_time = base::TimeTicks::Now();
+  SessionIOS* session = [self loadSessionFromPath:sessionPath];
+  UmaHistogramTimes("Session.WebStates.ReadFromFileTime",
+                    base::TimeTicks::Now() - start_time);
+  return session;
 }
 
 - (SessionIOS*)loadSessionFromPath:(NSString*)sessionPath {
@@ -281,12 +287,15 @@ NSString* const kRootObjectKey = @"root";  // Key for the root object.
   NSDataWritingOptions options =
       NSDataWritingAtomic | NSDataWritingFileProtectionComplete;
 
+  base::TimeTicks start_time = base::TimeTicks::Now();
   if (![sessionData writeToFile:sessionPath options:options error:&error]) {
     NOTREACHED() << "Error writing session file: "
                  << base::SysNSStringToUTF8(sessionPath) << ": "
                  << base::SysNSStringToUTF8([error description]);
     return;
   }
+  UmaHistogramTimes("Session.WebStates.WriteToFileTime",
+                    base::TimeTicks::Now() - start_time);
 }
 
 @end

tools/metrics/histograms/histograms.xml

@@ -131720,6 +131720,16 @@ should be kept until we remove incident reporting. -->
   </summary>
 </histogram>
 
+<histogram name="Session.WebStates.ReadFromFileTime" units="ms"
+    expires_after="2020-06-30">
+  <owner>justincohen@chromium.org</owner>
+  <owner>rohitrao@chromium.org</owner>
+  <summary>
+    The time needed to read iOS session webState data from a file. It's recorded
+    every time a webState is restored, typically on app startup.
+  </summary>
+</histogram>
+
 <histogram name="Session.WebStates.SerializedSize" units="KB"
     expires_after="2020-06-30">
   <owner>justincohen@chromium.org</owner>
@@ -131731,6 +131741,17 @@ should be kept until we remove incident reporting. -->
   </summary>
 </histogram>
 
+<histogram name="Session.WebStates.WriteToFileTime" units="ms"
+    expires_after="2020-06-30">
+  <owner>justincohen@chromium.org</owner>
+  <owner>rohitrao@chromium.org</owner>
+  <summary>
+    The time needed to write iOS session webState data to a file. It's recorded
+    every time the file is written to storage, which happens on page loads, tab
+    changes and app backgrounding.
+  </summary>
+</histogram>
+
 <histogram name="SessionCrashed.Bubble" enum="SessionCrashedBubbleUserAction"
     expires_after="M77">
   <owner>yiyaoliu@chromium.org</owner>