[ios] Remove expired metric IOS.DragAndDrop.DragContent

This was originally created in Aug 2017 in this CL:
https://chromium-review.googlesource.com/603690

This metric is no longer being used and has already expired
9 months ago.

Per go/histogram-expiry-guidelines:
"If the histogram is not in use now,
but might be useful in the far future, remove it."

Change-Id: I6903617fcfaf8ca3d9c46433386d7ccca508105f
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2580697
Auto-Submit: edchin <edchin@chromium.org>
Commit-Queue: Rohit Rao <rohitrao@chromium.org>
Reviewed-by: Robert Kaplow <rkaplow@chromium.org>
Reviewed-by: Rohit Rao <rohitrao@chromium.org>
Cr-Commit-Position: refs/heads/master@{#835326}

ios/chrome/app/chrome_overlay_window.mm

@@ -6,7 +6,6 @@
 
 #include "base/check.h"
 #import "ios/chrome/browser/crash_report/crash_keys_helper.h"
-#import "ios/chrome/browser/metrics/drag_and_drop_recorder.h"
 #import "ios/chrome/browser/metrics/size_class_recorder.h"
 #import "ios/chrome/browser/metrics/user_interface_style_recorder.h"
 #import "ios/chrome/browser/ui/util/ui_util.h"
@@ -20,7 +19,6 @@
     UserInterfaceStyleRecorder* userInterfaceStyleRecorder API_AVAILABLE(
         ios(13.0));
 @property(nonatomic, strong) SizeClassRecorder* sizeClassRecorder;
-@property(nonatomic, strong) DragAndDropRecorder* dragAndDropRecorder;
 
 // Initializes the size class recorder. On iPad It starts tracking horizontal
 // size class changes.
@@ -39,7 +37,6 @@
     // When not created via a nib, create the recorders immediately.
     [self initializeSizeClassRecorder];
     [self updateBreakpad];
-    _dragAndDropRecorder = [[DragAndDropRecorder alloc] initWithView:self];
     if (@available(iOS 13, *)) {
       _userInterfaceStyleRecorder = [[UserInterfaceStyleRecorder alloc]
           initWithUserInterfaceStyle:self.traitCollection.userInterfaceStyle];

ios/chrome/browser/metrics/BUILD.gn

@@ -180,8 +180,6 @@ source_set("metrics_browser_agent") {
 source_set("metrics_internal") {
   configs += [ "//build/config/compiler:enable_arc" ]
   sources = [
-    "drag_and_drop_recorder.h",
-    "drag_and_drop_recorder.mm",
     "first_user_action_recorder.cc",
     "first_user_action_recorder.h",
     "new_tab_page_uma.h",

ios/chrome/browser/metrics/drag_and_drop_recorder.h

-// Copyright 2017 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#ifndef IOS_CHROME_BROWSER_METRICS_DRAG_AND_DROP_RECORDER_H_
-#define IOS_CHROME_BROWSER_METRICS_DRAG_AND_DROP_RECORDER_H_
-
-#import <UIKit/UIKit.h>
-
-// Reports metrics for the drag and drop events on iOS 11+ for a given view.
-// Should be attached to the top most UIWindow to not miss any event.
-@interface DragAndDropRecorder : NSObject
-
-// Default initializer. Does not retain |view|.
-- (instancetype)initWithView:(UIView*)view NS_DESIGNATED_INITIALIZER;
-
-- (instancetype)init NS_UNAVAILABLE;
-
-@end
-
-#endif  // IOS_CHROME_BROWSER_METRICS_DRAG_AND_DROP_RECORDER_H_

ios/chrome/browser/metrics/drag_and_drop_recorder.mm

-// Copyright 2017 The Chromium Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#import "ios/chrome/browser/metrics/drag_and_drop_recorder.h"
-
-#include "base/metrics/histogram_macros.h"
-
-#if !defined(__has_feature) || !__has_feature(objc_arc)
-#error "This file requires ARC support."
-#endif
-
-namespace {
-
-// Reported metrics for the content of drag events.
-enum DragContentForReporting {
-  UNKNOWN = 0,
-  IMAGE = 1,
-  URL = 2,
-  TEXT = 3,
-  COUNT
-};
-
-// Records metrics regarding the content of drag and drops.
-void RecordDragAndDropContentHistogram(DragContentForReporting sample) {
-  // Wrapping the macro in a function because it expands to a lot of code and
-  // is called from multiple places.
-  UMA_HISTOGRAM_ENUMERATION("IOS.DragAndDrop.DragContent", sample,
-                            DragContentForReporting::COUNT);
-}
-
-// Records the DragAndDrop.DragContent histogram for a given |dropSession|.
-void RecordDragTypesForSession(id<UIDropSession> dropSession)
-    API_AVAILABLE(ios(11.0)) {
-  // Map keys must conform to the NSCopying protocol. Class doesn't declare
-  // that it does this, but Class does implement |copyWithZone:|, so the cast
-  // is safe.
-  static NSDictionary* classToSampleNameMap = @{
-    (id)[UIImage class] : @(DragContentForReporting::IMAGE),
-    (id)[NSURL class] : @(DragContentForReporting::URL),
-    (id)[NSString class] : @(DragContentForReporting::TEXT)
-  };
-  bool containsAKnownClass = false;
-  // Report a histogram for every item contained in |dropSession|.
-  for (Class klass in classToSampleNameMap) {
-    if ([dropSession canLoadObjectsOfClass:klass]) {
-      RecordDragAndDropContentHistogram(static_cast<DragContentForReporting>(
-          [classToSampleNameMap[klass] intValue]));
-      containsAKnownClass = true;
-    }
-  }
-  if (!containsAKnownClass) {
-    RecordDragAndDropContentHistogram(DragContentForReporting::UNKNOWN);
-  }
-}
-
-}  // namespace
-
-@interface DragAndDropRecorder ()<UIDropInteractionDelegate> {
-  // The currently active drop sessions.
-  NSHashTable* _dropSessions;
-}
-@end
-
-@implementation DragAndDropRecorder
-
-- (instancetype)initWithView:(UIView*)view {
-  self = [super init];
-  if (self) {
-    _dropSessions = [NSHashTable weakObjectsHashTable];
-    UIDropInteraction* dropInteraction =
-        [[UIDropInteraction alloc] initWithDelegate:self];
-    [view addInteraction:dropInteraction];
-  }
-  return self;
-}
-
-- (BOOL)dropInteraction:(UIDropInteraction*)interaction
-       canHandleSession:(id<UIDropSession>)session API_AVAILABLE(ios(11.0)) {
-  // |-dropInteraction:canHandleSession:| can be called multiple times for the
-  // same drop session.
-  // Maintain a set of weak references to these sessions to make sure metrics
-  // are recorded only once per drop session.
-  if (![_dropSessions containsObject:session]) {
-    [_dropSessions addObject:session];
-    RecordDragTypesForSession(session);
-  }
-  // Return "NO" as the goal of this UIDropInteractionDelegate is to report
-  // metrics and not intercept events.
-  return NO;
-}
-
-@end

tools/metrics/histograms/enums.xml

@@ -19605,6 +19605,9 @@ Called by update_document_policy_enum.py.-->
 </enum>
 
 <enum name="DragContent">
+  <obsolete>
+    Deprecated 12/2020.
+  </obsolete>
   <int value="0" label="Unknown"/>
   <int value="1" label="Image"/>
   <int value="2" label="URL"/>

tools/metrics/histograms/histograms_xml/ios/histograms.xml

@@ -281,6 +281,9 @@ reviews. Googlers can read more about this at go/gwsq-gerrit.
 
 <histogram name="IOS.DragAndDrop.DragContent" enum="DragContent"
     expires_after="2020-03-01">
+  <obsolete>
+    Removed 12/2020 as it is no longer needed for analysis.
+  </obsolete>
   <owner>jif@chromium.org</owner>
   <summary>
     The type of content that the user is dragging into Chrome. Because a drag