Chrome_Win: Renderer Crash - WebCore::FrameView::setFrameRect

This crash is caused by r166295
ismainframe check needs to be added before using the page

BUG=340580

Review URL: https://codereview.chromium.org/151363003

git-svn-id: svn://svn.chromium.org/blink/trunk@166431 bbb929c8-8fbe-4397-9dbb-9b2b20218538