Rename PartitionPageHeader to PartitionPage.

Simple rename; no functional change. We're in the process of moving partition
metadata out of line, therefore it is no longer appropriate to call it a
"Header".

BUG=324960
R=tsepez@chromium.org

Review URL: https://codereview.chromium.org/100503002

git-svn-id: svn://svn.chromium.org/blink/trunk@163050 bbb929c8-8fbe-4397-9dbb-9b2b20218538