Change the forced clear order of CPDF_DocPageData::Clear, and remove m_pDocument in CPDF_Color

If not forced clear, the counter of CPDF_Pattern will take care the count of referenced instance.
When forced clear, clear color first to make sure pattern still exists when referencing it.

BUG=411154
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/554913003