Revert "Move ParseAssignmentExpression to ParserBase."

This reverts revision 19908.

Reason: clang doesn't like it.

BUG=

Review URL: https://codereview.chromium.org/199233003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@19909 ce2b1a6d-e550-0410-aec6-3dcde31c8c00