embedded crosswalk webview doesn't include brotli in accept encoding request header.

[d159159]

Hi.
embedded crosswalk webview doesn't include brotli in "Accept-Encoding" header.
although he support decoding it. I test it by overwrite "Accept-Encoding" with "gzip, deflate, br". in loadurl method.
and he decode brotli compressed response successfully.
problem is the method I use, only apply for first request not the loaded resource like css and js files.
I modified crosswalk source but no success. same issue.