When encoding a string containing nul bytes base::JSONWriter will emit
the Unicode escape sequence "\u0000" which is valid JSON.
base::JSONReader however supports the nonstandard "\x00" escape sequence
but will silently drop the character "\u0000". This patch fixes
JSONParser::DecodeUTF16() properly append nul bytes to the output
string.

BUG=268991

Review-Url: https://codereview.chromium.org/2823023002
Cr-Commit-Position: refs/heads/master@{#465018}