fix(snack-bar): add explicit box-sizing

[crisbeto]

Fixes the snack bar not being readable in the cases where box-sizing: border-box is applied globally.

Fixes #1412.

Note: this can also be fixed by removing the explicit height, however I wasn't sure whether it wasn't there on purpose (e.g. to prevent line wraps).

[fxck]

Removing explicit height messes up alignment. But honestly, box-sizing: border-box should be explicitly set for all of the components and they should be styled with that in mind. Rather than having different box sizing for each component.

[crisbeto]

Hopefully this wouldn't be too much of a problem. In Material 1, there's only 2-3 cases total where the box sizing needs to be reset, I don't expect it to be much more for Material 2. This can also be fixed by removing the padding, but then the vertical alignment is a little trickier since it'd have to use line-height which isn't very consistent across browsers.

[jelbourn]

LGTM

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