Add support for interface builder to MDCSlider.

Summary:
Move common initialization to commonInit.
Add initWithCoder method.

Reviewers: junius, #material_components_ios_owners, cjcox

Reviewed By: #material_components_ios_owners, cjcox

Subscribers: cjcox

Projects: #material_components_ios

Differential Revision: http://codereview.cc/D131

Author: Ian Gordon

components/Slider/src/MDCSlider.m

@@ -36,35 +36,47 @@ @implementation MDCSlider {
 - (instancetype)initWithFrame:(CGRect)frame {
   self = [super initWithFrame:frame];
   if (self) {
-    CGRect trackFrame = CGRectInset(frame, 8.f, 0.f);
-
-    _thumbTrack = [[MDCThumbTrack alloc] initWithFrame:trackFrame onTintColor:[[self class] defaultColor]];
-    _thumbTrack.delegate = self;
-    _thumbTrack.disabledTrackHasThumbGaps = YES;
-    _thumbTrack.trackEndsAreInset = YES;
-    _thumbTrack.thumbRadius = kSliderThumbRadius;
-    _thumbTrack.thumbMaxRippleRadius = kSliderThumbMaxRippleRadius;
-    _thumbTrack.trackOffColor = [[self class] defaultTrackOffColor];
-    [_thumbTrack addTarget:self
-                    action:@selector(thumbTrackValueChanged:)
-          forControlEvents:UIControlEventValueChanged];
-    [_thumbTrack addTarget:self
-                    action:@selector(thumbTrackTouchDown:)
-          forControlEvents:UIControlEventTouchDown];
-    [_thumbTrack addTarget:self
-                    action:@selector(thumbTrackTouchUpInside:)
-          forControlEvents:UIControlEventTouchUpInside];
-    [_thumbTrack addTarget:self
-                    action:@selector(thumbTrackTouchUpOutside:)
-          forControlEvents:UIControlEventTouchUpOutside];
-    [_thumbTrack addTarget:self
-                    action:@selector(thumbTrackTouchCanceled:)
-          forControlEvents:UIControlEventTouchCancel];
-    [self addSubview:_thumbTrack];
+    [self commonInit];
   }
   return self;
 }
 
+- (instancetype)initWithCoder:(NSCoder *)aDecoder {
+  self = [super initWithCoder:aDecoder];
+  if (self) {
+    [self commonInit];
+  }
+  return self;
+}
+
+- (void)commonInit {
+  CGRect trackFrame = CGRectInset(self.frame, 8.f, 0.f);
+
+  _thumbTrack = [[MDCThumbTrack alloc] initWithFrame:trackFrame onTintColor:[[self class] defaultColor]];
+  _thumbTrack.delegate = self;
+  _thumbTrack.disabledTrackHasThumbGaps = YES;
+  _thumbTrack.trackEndsAreInset = YES;
+  _thumbTrack.thumbRadius = kSliderThumbRadius;
+  _thumbTrack.thumbMaxRippleRadius = kSliderThumbMaxRippleRadius;
+  _thumbTrack.trackOffColor = [[self class] defaultTrackOffColor];
+  [_thumbTrack addTarget:self
+                  action:@selector(thumbTrackValueChanged:)
+        forControlEvents:UIControlEventValueChanged];
+  [_thumbTrack addTarget:self
+                  action:@selector(thumbTrackTouchDown:)
+        forControlEvents:UIControlEventTouchDown];
+  [_thumbTrack addTarget:self
+                  action:@selector(thumbTrackTouchUpInside:)
+        forControlEvents:UIControlEventTouchUpInside];
+  [_thumbTrack addTarget:self
+                  action:@selector(thumbTrackTouchUpOutside:)
+        forControlEvents:UIControlEventTouchUpOutside];
+  [_thumbTrack addTarget:self
+                  action:@selector(thumbTrackTouchCanceled:)
+        forControlEvents:UIControlEventTouchCancel];
+  [self addSubview:_thumbTrack];
+}
+
 #pragma mark - ThumbTrack passthrough methods
 
 - (void)setTrackBackgroundColor:(UIColor *)trackBackgroundColor {