Reviewed by Darin Adler.

        Assertion when loading two URLs in quick succession
        https://bugs.webkit.org/show_bug.cgi?id=52649
        <rdar://problem/8764645>

        If beginLoad is called twice without didStartProvisionalLoad being called in between,
        m_pendingProvisionalSandboxExtension would be non-null and an assert would fire.

        * WebProcess/WebPage/WebPage.cpp:
        (WebKit::WebPage::SandboxExtensionTracker::beginLoad):
        Instead of asserting that m_pendingProvisionalSandboxExtension is null, invalidate it
        if it is not null, since that's valid.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@76046 268f45cc-cd09-0410-ab3c-d52691b4dbfc