Fix bug in CPDF_PageContentGenerate constructor and destructor

m_pageObjects never gets initialize, thus making CPDF_PageContentGenerate::GenerateContent() doing nothing.

Since the CPFD_PageObject are owned by m_pPage, no need to release them in the destructor.

BUG=385119
R=thestig@chromium.org, vitalybuka@chromium.org

Review URL: https://codereview.chromium.org/470253004