[BottomNav] Reduce default font to caption (material-components#2799)

BottomNavigation previously had a nil default font, so a default
`button` style font was set. This is too large (and is Medium weight),
so the font should be changed to something smaller. 'Caption' style
looks better.

components/BottomNavigation/src/MDCBottomNavigationBar.m

@@ -173,7 +173,7 @@ - (void)commonMDCBottomNavigationBarInit {
   [self addSubview:_containerView];
   [self setElevation:kMDCBottomNavigationBarElevation];
   _itemViews = [NSMutableArray array];
-  _itemTitleFont = [UIFont mdc_standardFontForMaterialTextStyle:MDCFontTextStyleButton];
+  _itemTitleFont = [UIFont mdc_standardFontForMaterialTextStyle:MDCFontTextStyleCaption];
 }
 
 - (void)layoutSubviews {