Make CSSMin::getMimeType public rather than protected

There's no reason why it should be protected. People sometimes use
'protected' to mean 'ancillary to the main purpose of the class', but
'protected' comes with the added cost of making the class API less
flexible. I intend to call this method in a future revision of Id052a04dd2,
but I think it should be public regardless.

Change-Id: I56365df3152df1180ce73de98202e3a91def1485
Merge "Make CSSMin::getMimeType public rather than protected"