Record filesystem selected when formatting to UMA

Bug: 632988
Change-Id: Ia4c1b0020958ba8e4243f4797945fc05d2c89400
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1706457
Reviewed-by: Steven Holte <holte@chromium.org>
Reviewed-by: Anand Mistry <amistry@chromium.org>
Commit-Queue: Austin Tankiang <austinct@chromium.org>
Cr-Commit-Position: refs/heads/master@{#679847}

chromeos/disks/disk_mount_manager.cc

@@ -19,6 +19,7 @@
 #include "base/macros.h"
 #include "base/memory/ptr_util.h"
 #include "base/memory/weak_ptr.h"
+#include "base/metrics/histogram_functions.h"
 #include "base/observer_list.h"
 #include "base/strings/string_util.h"
 #include "chromeos/constants/chromeos_features.h"
@@ -555,6 +556,9 @@ class DiskMountManagerImpl : public DiskMountManager,
     DiskMap::const_iterator disk = disks_.find(device_path);
     DCHECK(disk != disks_.end() && disk->second->mount_path().empty());
 
+    base::UmaHistogramEnumeration("FileBrowser.FormatFileSystemType",
+                                  filesystem);
+
     const std::string filesystem_str = FormatFileSystemTypeToString(filesystem);
     pending_format_changes_[device_path] = {filesystem_str, label};
 

chromeos/disks/disk_mount_manager.h

@@ -28,11 +28,14 @@ enum MountCondition {
 };
 
 // Possible filesystem types that can be passed to FormatMountedDevice.
+// These values are persisted to logs. Entries should not be renumbered and
+// numeric values should never be reused.
 enum class FormatFileSystemType {
   kUnknown = 0,
   kVfat = 1,
   kExfat = 2,
   kNtfs = 3,
+  kMaxValue = kNtfs,
 };
 
 // This class handles the interaction with cros-disks.

tools/metrics/histograms/enums.xml

@@ -25025,6 +25025,13 @@ Called by update_net_error_codes.py.-->
   <int value="2" label="Unable to open"/>
 </enum>
 
+<enum name="FileManagerFormatFileSystemType">
+  <int value="0" label="Unknown"/>
+  <int value="1" label="FAT32"/>
+  <int value="2" label="exFAT"/>
+  <int value="3" label="NTFS"/>
+</enum>
+
 <enum name="FileManagerListType">
   <int value="0" label="Uninitialized"/>
   <int value="1" label="List view (detail)"/>

tools/metrics/histograms/histograms.xml

@@ -43964,6 +43964,15 @@ uploading your change for review.
   </summary>
 </histogram>
 
+<histogram name="FileBrowser.FormatFileSystemType"
+    enum="FileManagerFormatFileSystemType" expires_after="2019-12-06">
+  <owner>austinct@chromium.org</owner>
+  <summary>
+    Chrome OS File Browser: this records the filesystem selected when formatting
+    an external drive.
+  </summary>
+</histogram>
+
 <histogram name="FileBrowser.HardUnpluggedAroundSuspend.TimeSinceResume"
     units="ms" expires_after="M79">
   <obsolete>