Call toWebCoreAtomicString() in V8Window::namedSecurityCheckCustom()

Call toWebCoreAtomicString() in V8Window::namedSecurityCheckCustom() instead
of toWebCoreString(). This avoids uselessly creating a String and then
convert it to an AtomicString since FrameTree::scopedChild() takes an
AtomicString in argument.

R=haraken
BUG=323739

Review URL: https://codereview.chromium.org/90153002

git-svn-id: svn://svn.chromium.org/blink/trunk@162731 bbb929c8-8fbe-4397-9dbb-9b2b20218538