fix(demo): fix centering of MDL icons
fix(demo): fix centering of MDL icons

Some icons on the demo were not centered correctly, such as the custom
content tab's "plus" button.  Some conflicting styles were changing
the line-height.

Change-Id: I08c9bb01642346b1a3b1162f779300221ae23246

shaka-project/shaka-player,shaka-project/shaka-player,shaka-project/shaka-player,tvoli/shaka-player,tvoli/shaka-player,tvoli/shaka-player,shaka-project/shaka-player,tvoli/shaka-player