Made return types of various Value::DeepCopy() implementations more specific

Made return types of various Value::DeepCopy() implementations more specific

Since C++ supports covariant return types, a subclass of Value can return its
own pointer type for Value::DeepCopy().

Also made signatures of Value::Create*Value() more specific.

Removed now-unnecessary casts.

Added test for covariant return types.

BUG=None
TEST=Compile

Committed: http://src.chromium.org/viewvc/chrome?view=rev&revision=71617

[akalin, 2011-01-16 11:23:40]

+evmar for review

[evanm, 2011-01-16 18:23:57]

Didn't realize that you could override in this way.  LGTM if you're sure the
overrides work

[akalin, 2011-01-16 22:09:36]

On 2011/01/16 18:23:57, evanm wrote:
> Didn't realize that you could override in this way.  LGTM if you're sure the
> overrides work

I'm sure, but I added a test anyway, since DeepCopy() isn't pure virtual in
Values.

Committing as soon as trybots pass.