[WebSocket] Compress an empty string to one byte.

Currently permessage-deflate deflater compresses an empty string to a
two-byte string "\x02\x00". It should be "\x00" to save space.

BUG=337781
R=tyoshino

Review URL: https://codereview.chromium.org/141613004

git-svn-id: svn://svn.chromium.org/blink/trunk@165758 bbb929c8-8fbe-4397-9dbb-9b2b20218538