Real fix for <rdar://problem/9025723> CrashTracer: [USER]

Real fix for <rdar://problem/9025723> CrashTracer: [USER]
1 crash in WebProcess at com.apple.WebKit2:
WebKit::WebPage::didChangeScrollOffsetForMainFrame + 31

Reviewed by Adele Peterson.

* WebProcess/WebCoreSupport/WebFrameLoaderClient.cpp:
(WebKit::WebFrameLoaderClient::didChangeScrollOffset):
Add null check for the FrameView. This function can be called when the
FrameView is being torn down during a transition to a new FrameView.

* WebProcess/WebPage/WebPage.cpp:
(WebKit::WebPage::didChangeScrollOffsetForMainFrame):
Revert last attempt.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@79064 268f45cc-cd09-0410-ab3c-d52691b4dbfc