Fix crash due RegExpAtom method called on RegExpCharacterClass object.

Fix crash due RegExpAtom method called on RegExpCharacterClass object.

In the RegExpUnparser::VisitText(RegExpText* that, void* data) function always RegExpUnparser::VisitAtom function called via that->elements()->at(i).data.u_atom->Accept(this, data); even if the type of the object is RegExpCharacterClass.

The problem shows using g++ 4.7(.2, .3) since r16232, since GCC optimizes virtual method calls to direct calls based on __final/final hints. Tested on MIPS and x64:
Program received signal SIGSEGV, Segmentation fault.
0x0000000000588928 in v8::internal::RegExpUnparser::VisitAtom(v8::internal::RegExpAtom*, void*) ()

This cleans up the TextElement class to avoid the unsafe+unchecked union access, that caused the crash.

TEST=cctest/test-regexp/ParserRegression
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/22815033

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@16289 ce2b1a6d-e550-0410-aec6-3dcde31c8c00