DevTools: [CSS] support different cssrule types in InspectorStyleSheet

The patch adds flattenning and source data collecting of the following
CSSRules:
- charset rule
- page rule
- font-face rule
- viewport rule

The charset rule was lacking source data generation, so the bison grammer
had to be changed in order to support it.

BUG=417778
R=apavlov

Review URL: https://codereview.chromium.org/606023002

git-svn-id: svn://svn.chromium.org/blink/trunk@183058 bbb929c8-8fbe-4397-9dbb-9b2b20218538