Add a method to read a stream without advancing it.

Add a method to read a stream without advancing it.

Add a virtual method on SkStream which will do a "peek" some bytes, so
that those bytes are read, but the next call to read will be
unaffected.

Implement peek for SkMemoryStream, where the implementation is simple
and obvious.

Implement peek on SkFrontBufferedStream.

Add tests.

Motivated by decoding streams which cannot be rewound.

TBR=reed@google.com

BUG=skia:3257

Review URL: https://codereview.chromium.org/1044953002