Rename LChunkBase to LChunk, LChunk to LPlatformChunk and remove some unneeded explicit constructor attributes.

BUG=
TEST=

Review URL: https://chromiumcodereview.appspot.com/10701157

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@12067 ce2b1a6d-e550-0410-aec6-3dcde31c8c00