3.0.0 hotfix: make contentView writable.

Summary: This resolves a breakage that was expecting the setContentView: API to exist.

Reviewers: #mdc_ios_owners, iangordon, ajsecord

Reviewed By: #mdc_ios_owners, iangordon, ajsecord

Projects: #material_components_ios

Differential Revision: http://codereview.cc/D485

Author: Jeff Verkoeyen

components/FlexibleHeader/src/MDCFlexibleHeaderView.h

@@ -94,7 +94,7 @@ typedef NS_ENUM(NSInteger, MDCFlexibleHeaderScrollPhase) {
 /**
  Content for the flexible header should be added to the content view.
  */
-@property(nonatomic, retain, nonnull, readonly) UIView *contentView;
+@property(nonatomic, retain, nonnull) UIView *contentView;
 
 /**
  Sets a custom shadow layer and a block that should be executed when shadow intensity changes.