Clarify Value::Get*()'s interface to reflect the current implementation.

The fact that each Get methods can be used with a NULL out-value simply to check that a value exists (and has the desired type) under a given key is used all over chrome, but isn't actually documented so this simply makes it official by documenting and testing this fact.

Also document the fact that GetDouble will succeed for TYPE_INTEGER as well as TYPE_DOUBLE (which was a surprise to me when I wrote the tests for this CL).

Review URL: https://codereview.chromium.org/191983002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@255834 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 255834