[MDCButton] Ink should match button frame to be centered properly

Summary:
Before/after screenshot: http://codereview.cc/M30

Addresses issue:
https://github.com/google/material-components-ios/issues/475

Reviewers: ajsecord, randallli, #mdc_ios_owners

Reviewed By: randallli, #mdc_ios_owners

Projects: #material_components_ios

Differential Revision: http://codereview.cc/D805

components/Buttons/src/MDCButton.m

@@ -275,10 +275,11 @@ - (void)layoutSubviews {
   self.layer.shadowPath = [self boundingPath].CGPath;
   self.layer.cornerRadius = [self cornerRadius];
 
-  // Calculate center based on contentEdgeInsets
-  _inkView.usesCustomInkCenter = YES;
+  // The ink view frame should match the content rect to be centered correctly. The content rect is
+  // calculated based on the title, image size and padding; then adjusted based on the control
+  // content alignment. The button frame cannot be used since it has rect offsets without insets.
   CGRect contentRect = [self contentRectForBounds:self.bounds];
-  _inkView.customInkCenter = CGPointMake(CGRectGetMidX(contentRect), CGRectGetMidY(contentRect));
+  _inkView.frame = contentRect;
 }
 
 - (BOOL)pointInside:(CGPoint)point withEvent:(UIEvent *)event {