Reviewed by Darin Adler.

        Extract a function to process contents for one node from Range::processContents
        https://bugs.webkit.org/show_bug.cgi?id=54282

        Extracted Range::processContentsBetweenOffsets, which process contents of a node
        between two offsets. This function is used for the simple case where the start
        and the end containers are of the same node, and to process start and end containers
        in the complex case.

        When the function takes a non-null fragment (simple case), it appends the processed
        contents to the fragment; character data and processing instruction's contents are copied
        between the offsets, and descendants are copied for node of other types (not node itself).

        When the fragment is null (complex case), the function copies contents of character data,
        processing instruction, and node of other types (including node itself).

        No new tests are added since this is a refactoring.

        * dom/Range.cpp:
        (WebCore::highestAncestorUnderCommonRoot): Added.
        (WebCore::Range::processContents): Calls highestAncestorUnderCommonRoot and
        processContentsBetweenOffsets.
        (WebCore::Range::processContentsBetweenOffsets): Added.
        * dom/Range.h:


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@78409 268f45cc-cd09-0410-ab3c-d52691b4dbfc