md-list-icon is not aligned with global `box-sizing: inherit`

[willshowell]

Bug, feature request, or proposal:

Bug

It's pretty common to globally apply inherited box sizing to all elements:

html {
  box-sizing: border-box;
}
*, *:before, *:after {
  box-sizing: inherit;
}

This causes the md-list-icon to no longer keep the correct size, because it is inheriting border-box from .mat-list-item-content. It should explicitly use content-box.

What are the steps to reproduce?

http://plnkr.co/edit/nDwtfSsqTrotyZpNfr5I?p=preview

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