Streamline creating a new window a bit.

Don't require a NavigationAction parameter on ChromeClient::createWindow, as
the only part of it being accessed is duplicated in FrameLoadRequest.

Don't set and reset request parameters in CreateWindow.cpp.

BUG=none

Review URL: https://chromiumcodereview.appspot.com/22849013

git-svn-id: svn://svn.chromium.org/blink/trunk@156329 bbb929c8-8fbe-4397-9dbb-9b2b20218538