Remove Japanese encoding detector

1. Remove Japanese encoding detector. Often, it misdetects. More importantly, it's not used at all (it's a dead code unless the default encoding is Japanese [1]). 
2. Try to sniff meta tag for charset before auto-detecting.

[1] That's because 'shouldAutoDetect' and isJapaneseEncoding() cannot be satisfied at the same time except when the default encoding is Japanese (e.g. Shift_JIS). 

The only downside is that we'll misdecode a page in ISO-2022-JP without any charset declaration (http C-T header, html meta, xml head, css , etc) even when the default encoding is Japanese. However, the percentage of ISO-2022-JP pages without charset declaration is miniscule and it's not worth having this code. 

BUG=305837
TEST=LayoutTest: fast/encoding/*

Review URL: https://codereview.chromium.org/26675006

git-svn-id: svn://svn.chromium.org/blink/trunk@159408 bbb929c8-8fbe-4397-9dbb-9b2b20218538