Reviewed by Darin Adler.

        Keep track of the latest update timestamp in the backing store
        https://bugs.webkit.org/show_bug.cgi?id=52848

        * Shared/UpdateInfo.h:
        (WebKit::UpdateInfo::UpdateInfo):
        Initialize timestamp to 0.

        * UIProcess/BackingStore.cpp:
        (WebKit::BackingStore::BackingStore):
        Initialize m_latestUpdateTimestamp to 0.

        (WebKit::BackingStore::incorporateUpdate):
        If the update is too old, discard it. Otherwise, create a bitmap
        and pass it to platformIncorporateUpdate. Finally update the timestamp.

        * UIProcess/BackingStore.h:
        Add m_latestUpdateTimestamp.

        * UIProcess/mac/BackingStoreMac.mm:
        (WebKit::BackingStore::platformIncorporateUpdate):
        Update now that we are already given the shareable bitmap.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@76305 268f45cc-cd09-0410-ab3c-d52691b4dbfc