Add test for new FrontBufferedStream behavior.
Add test for new FrontBufferedStream behavior.

Test for https://skia.googlesource.com/skia/+/dd5a1e094c19fa10202c37c50a1f799e5af5dac0

Verify that FrontBufferedStream does not attempt to read beyond the
end of its underlying stream.

Review URL: https://codereview.chromium.org/641813009

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