Remove EnsurePtrConvertibleType from RefPtr assignment operators

Using of EnsurePtrConvertibleType in assignment operators (unlike copy constructors) does not help to solve ambiguousness problem.
EnsurePtrConvertibleType would make sence there if we had different assignment operators implementations selected in compile time but this is not the case.

On the other hand EnsurePtrConvertibleType affects dramatically the readability of assignment operators code so it should be removed from there.

Review URL: https://chromiumcodereview.appspot.com/23694010

git-svn-id: svn://svn.chromium.org/blink/trunk@157073 bbb929c8-8fbe-4397-9dbb-9b2b20218538