There are two TryConvertDoubleToInt64 functions: one rounds, the other checks
if a double can be exactly represented as an int. This patch renames the second
instance to reflect its purpose more clearly.

BUG=
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/258933008

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@21020 ce2b1a6d-e550-0410-aec6-3dcde31c8c00