If the stream can peek less than requested, peek that amount. Return
the number of bytes peeked.
This simplifies crrev.com/1472123002. For a stream that is smaller than
14 bytes, it can successfully peek, meaning the client will not need to
fall back to read() + rewind(), which may fail if the stream can peek
but not rewind.
This CL revives code from patch set 3 of crrev.com/1044953002, where I
initially introduced peek() (including tests).
Add a test for SkFrontBufferedStream that verifies that peeking does
not make rewind() fail (i.e. by reading past the internal buffer).
BUG=skia:3257
Review URL: https://codereview.chromium.org/1490923005
d61c384342