Remove FontDescription::mutableFontDescription

Change RenderStyle::setWordSpacing and RenderStyle::setLetterSpacing to
replace the FontDescription instead of modifying the existing instance,
the same way the other font methods already do. This allows us to
remove the mutableFontDescription method from FontDescription.

R=dglazkov@chromium.org
BUG=

Review URL: https://codereview.chromium.org/207573002

git-svn-id: svn://svn.chromium.org/blink/trunk@169718 bbb929c8-8fbe-4397-9dbb-9b2b20218538