Disable most MathML attributes

The HTML parser only needs the encoding attribute to
parse MathML content, so do not generate the
other MathML attribute strings to save a bit in binary
size and startup time.

Review URL: https://codereview.chromium.org/574523002

git-svn-id: svn://svn.chromium.org/blink/trunk@182055 bbb929c8-8fbe-4397-9dbb-9b2b20218538