Remove clipping view (#5897)

### Context
In working on navigation drawer I noticed the addition of the clippingScrollView. This additional view just makes the component more complex with no known benefit. This change removes that view in order to simply the component and remove the extra subview in the hierarchy.
### The problem
Adding the clippingScrollView adds extra complexity to the navigation drawer.
### The fix
Remove the clipping scroll view.

components/NavigationDrawer/src/private/MDCBottomDrawerContainerViewController.m

@@ -128,9 +128,6 @@ @interface MDCBottomDrawerContainerViewController () <UIScrollViewDelegate>
 // The main scroll view.
 @property(nonatomic, readonly) UIScrollView *scrollView;
 
-// View that functions as the superview of |scrollView|. Used to clip the top of the scroll view.
-@property(nonatomic, readonly) UIView *scrollViewClippingView;
-
 // The top header bottom shadow layer.
 @property(nonatomic) MDCShadowLayer *headerShadowLayer;
 
@@ -153,7 +150,6 @@ - (CGFloat)calculateInitialDrawerFactor;
 
 @implementation MDCBottomDrawerContainerViewController {
   UIScrollView *_scrollView;
-  UIView *_scrollViewClippingView;
   CGFloat _contentHeaderTopInset;
   CGFloat _contentHeightSurplus;
   CGFloat _addedContentHeight;
@@ -394,8 +390,7 @@ - (void)viewDidLoad {
 
   self.view.backgroundColor = [UIColor clearColor];
 
-  [self.view addSubview:self.scrollViewClippingView];
-  [self.scrollViewClippingView addSubview:self.scrollView];
+  [self.view addSubview:self.scrollView];
 
   // Top header shadow layer starts as hidden.
   self.headerShadowLayer.hidden = YES;
@@ -425,12 +420,7 @@ - (void)viewWillLayoutSubviews {
 
   // Layout the clipping view and the scroll view.
   if (self.currentlyFullscreen) {
-    CGRect scrollViewClippingViewFrame = self.presentingViewBounds;
-    scrollViewClippingViewFrame.origin.y = self.topHeaderHeight;
-    scrollViewClippingViewFrame.size.height -= self.topHeaderHeight;
-    self.scrollViewClippingView.frame = scrollViewClippingViewFrame;
     CGRect scrollViewFrame = self.presentingViewBounds;
-    scrollViewFrame.origin.y = -self.topHeaderHeight;
     self.scrollView.frame = scrollViewFrame;
   } else {
     CGRect scrollViewFrame = self.presentingViewBounds;
@@ -439,7 +429,6 @@ - (void)viewWillLayoutSubviews {
           self.presentingViewBounds.size.height / 2;
       scrollViewFrame.size.height += heightSurplusForSpringAnimationOvershooting;
     }
-    self.scrollViewClippingView.frame = scrollViewFrame;
     self.scrollView.frame = scrollViewFrame;
   }
 
@@ -595,14 +584,12 @@ - (void)updateContentHeaderWithTransitionToTop:(CGFloat)headerTransitionToTop
     // is shown in fullscreen.
     [contentHeaderView removeFromSuperview];
     [self.view addSubview:contentHeaderView];
-    self.scrollViewClippingView.clipsToBounds = YES;
     [self.view setNeedsLayout];
   } else if (!self.currentlyFullscreen && contentHeaderView.superview != self.scrollView) {
     // The content header should be scrolled together with the rest of the content when the drawer
     // is not in fullscreen.
     [contentHeaderView removeFromSuperview];
     [self.scrollView addSubview:contentHeaderView];
-    self.scrollViewClippingView.clipsToBounds = NO;
     [self.view setNeedsLayout];
   }
   CGFloat contentHeaderViewWidth = self.presentingViewBounds.size.width;
@@ -637,14 +624,6 @@ - (UIScrollView *)scrollView {
   return _scrollView;
 }
 
-- (UIView *)scrollViewClippingView {
-  if (!_scrollViewClippingView) {
-    _scrollViewClippingView = [[UIView alloc] init];
-    _scrollViewClippingView.backgroundColor = [UIColor clearColor];
-  }
-  return _scrollViewClippingView;
-}
-
 - (CGFloat)contentHeaderTopInset {
   if (_contentHeaderTopInset == NSNotFound) {
     [self cacheLayoutCalculations];