Fix ios compilation with newer clang.

Class cannot be used as NSDictionary key.

Bug: 1017213
Change-Id: I1f660792ccf7ab648cf4fcde3be3b5b8a1b8d745
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1884671
Commit-Queue: Olivier Robin <olivierrobin@chromium.org>
Auto-Submit: Olivier Robin <olivierrobin@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#710044}

ios/chrome/browser/metrics/drag_and_drop_recorder.mm

@@ -32,10 +32,13 @@ void RecordDragAndDropContentHistogram(DragContentForReporting sample) {
 // 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 = @{
-        [UIImage class] : @(DragContentForReporting::IMAGE),
-        [NSURL class] : @(DragContentForReporting::URL),
-        [NSString class] : @(DragContentForReporting::TEXT)
+    (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|.