Implement handling of arrow functions in the parser

Arrow functions are parsed from ParseAssignmentExpression(). Handling the
parameter list is done by letting ParseConditionalExpression() parse a comma
separated list of identifiers, and it returns a tree of BinaryOperation nodes
with VariableProxy leaves, or a single VariableProxy if there is only one
parameter. When the arrow token "=>" is found, the VariableProxy nodes are
passed to ParseArrowFunctionLiteral(), which will then skip parsing the
paramaeter list. This avoids having to rewind when the arrow is found and
restart parsing the parameter list.

Note that the empty parameter list "()" is handled directly in
ParsePrimaryExpression(): after is has consumed the opening parenthesis,
if a closing parenthesis follows, then the only valid input is an arrow
function. In this case, ParsePrimaryExpression() directly calls
ParseArrowFunctionLiteral(), to avoid needing to return a sentinel value
to signal the empty parameter list. Because it will consume the body of
the arrow function, ParseAssignmentExpression() will not see the arrow
"=>" token as next, and return the already-parser expression.

The implementation is done in ParserBase, so it was needed to do some
additions to ParserBase, ParserTraits and PreParserTraits. Some of the
glue code can be removed later on when more more functionality is moved
to ParserBase.

Additionally, this adds a runtime flag "harmony_arrow_functions"
(disabled by default); enabling "harmony" will enable it as well.

BUG=v8:2700
LOG=N
R=marja@chromium.org

Review URL: https://codereview.chromium.org/383983002

Patch from Adrián Pérez de Castro <aperez@igalia.com>.

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22366 ce2b1a6d-e550-0410-aec6-3dcde31c8c00