rollback of thumbtrack weak/strong diff

Summary: This reverts commit 43123a4.

Test Plan: revert-hammer

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Author: randallli

components/private/ThumbTrack/src/MDCThumbTrack.m

@@ -621,11 +621,11 @@ - (void)setValueFromThumbPosition:(CGPoint)position isTap:(BOOL)isTap {
       userGenerated:YES
          completion:^{
            __strong typeof(weakSelf) strongSelf = weakSelf;
-           __strong typeof(strongSelf->_delegate) strongBlockDelegate = strongSelf->_delegate;
+           __strong typeof(strongSelf->_delegate) strongDelegate = strongSelf->_delegate;
            [strongSelf sendDiscreteChangeAction];
-           if (strongSelf && [strongBlockDelegate
+           if (strongSelf && [strongDelegate
                                  respondsToSelector:@selector(thumbTrack:didAnimateToValue:)]) {
-             [strongBlockDelegate thumbTrack:strongSelf didAnimateToValue:value];
+             [strongDelegate thumbTrack:strongSelf didAnimateToValue:value];
            }
          }];
 }