[Isolates] Fix auto extraction of isolate from heap objects in handle constructor.

[Isolates] Fix auto extraction of isolate from heap objects in handle constructor.

My previous attempt used Handle(T*) and Handle(HeapObject*) overloads,
but since T is a class template parameter (and not just a paramer of
the constructor) the first signature is preferred when the argument is
convertible to T*.

This patch exposed an issue with SaveContext which created a handle
pointing to NULL context.

Bonus: a few lint fixes.

Committed: http://code.google.com/p/v8/source/detail?r=5662

[yurys, 2010-10-19 12:43:52]

LGTM