We add a new class CSSVariableResolver whose job is to take the
inherited computed variables and the specified variable declarations and
to perform cycle removal and resolution of the variables, storing the
result in the CSSVariableValues object on an nsStyleVariables. We use
CSSVariableResolver in nsRuleNode::ComputeVariablesData.
The variable resolver does this:
1. Asks the CSSVariableValues and CSSVariableDeclarations objects
to add their variables to it.
2. Calls in to a new nsCSSParser function
EnumerateVariableReferences that informs the resolver which
other variables a given variable references, and by doing so,
builds a graph of variable dependencies.
3. Removes variables involved in cyclic references using Tarjan's
strongly connected component algorithm, setting those variables
to have an invalid value.
4. Calls in to a new nsCSSParser function ResolveVariableValue
to resolve the remaining valid variables by substituting variable
references.
We extend nsCSSParser::ParseValueWithVariables to take a callback
function to be invoked when encountering a variable reference. This
lets EnumerateVariableReferences re-use ParseValueWithVariables.
CSSParserImpl::ResolveValueWithVariableReferences needs different
error handling behaviour from ParseValueWithVariables, so we don't
re-use it.
CSSParserImpl::AppendImpliedEOFCharacters is used to take the
value returned from nsCSSScanner::GetImpliedEOFCharacters while
resolving variable references that were declared using custom
properties that encountered EOF before being closed properly.
The SeparatorRequiredBetweenTokens helper function in nsCSSParser.cpp
implements the serialization rules in CSS Syntax Module Level 3:
https://dvcs.w3.org/hg/csswg/raw-file/3479cdefc59a/css-syntax/Overview.html#serialization
