Remove runtime flag check assert on public method.
WebFrame::sendOrientationChangeEvent is public method and could be called without runtime flag enabled.

BUG=335258

Review URL: https://codereview.chromium.org/141303002

git-svn-id: svn://svn.chromium.org/blink/trunk@165293 bbb929c8-8fbe-4397-9dbb-9b2b20218538