Remove custom code for JavaScriptCallFrame.setVariableValue() operation

Remove custom code for JavaScriptCallFrame.setVariableValue() operation by using
ScriptValue instead of v8::Value on the implementation side and by marking
the arguments as optional to behave the same way.

TEST=inspector-protocol/debugger/debugger-setVariableValue.html

Review URL: https://codereview.chromium.org/67373003

git-svn-id: svn://svn.chromium.org/blink/trunk@161831 bbb929c8-8fbe-4397-9dbb-9b2b20218538